[P3D]

My take on Gath - based on SP experience.

Units:
Most of them has wasteland survival, some have fire resistance.

Reubenite archer - standard shortbow archer, needs 7 resources so difficult to mass. But has a mapmove of 2. I'd recruit it instead of the slingers.
Benjaminite Slinger - with a precision of 12 roughly archer-equivalent. Has two shortwords but this units should not get into melee. Same protection as the archer, but more expensive.
Naphtali spearman: Light infantry (prot 9) with javelins and shields. If sacred, they'd be a great unit, but not so.
Zebulunite Soldier: Light-medium infantry with spears only and good (13) morale. Honestly, no reason to recruit them, ever.
Hornblower: buy a few of them if you have a lot of non-sacred infantry - or your Levites face awe/fearful troops.
Gadite: Heavy infantry (prot 16) with shortsword and Javelin. Mapmove 2, fire resistance, wasteland survival, 11 morale. You'd recruit them if they were not 27 resource or you'd not have Levites. Strength of 11 is good for both the shortsword and javelins.
Asherite: Nice broadsword heavies with a mapmove of 1. And has the same damage as the Gadite. Has Tower Shield but not much other reasons to buy it.
Levite: A mediocre medium infantry, but being sacred not a bad unit. Great morale (14 without bless). The main reason to recruit them is the reduced upkeep cost, but with your H3 priests they are almost always blessed in combat. Your main priority to buy in secondary castles - decent sized army without crippling maintenance costs. But need other units to do the slaughter. As a bonus, good patroller.
Gittite: 30g/25r giant medium infantry with Javelins and spear, 15 strength for 18 damage. If you have gold, better than than the other javelineers but about three times as expensive to maintain. Fire resistance.
Gibbor: 75g/39r sacred giant. 38HP (good) size 4, so you should mix it with Levites if they are on hold/attack. Magic sword to damage enemy SCs who thought etherealness had been a good idea. And has 2.5gold maintenace vs. 2 of the Gittite. I recruit them in the capital until I have resources are out, then switch to levites. Will accumulate 2 stars in no time. Has Gluttony so prepare to buy Bags of Wine, Cauldrons and Summer Swords if you have a lot of them.

Leaders:
Iassacharite scout: has a shortbow. You won't buy it.
Iassacharite sage: one of the main strength of Gath if played correctly. 50GP for 3RP (plus 10% S1) would not be bad if the unit would not be SACRED to top it. Granted, you need an overpriced 800GP temple to recruit them, but the reduced army maintenance from your sacred army leaves you some gold. Can be recruited without a lab if you are content to walk them 2-3 provinces to start researching. With Magic-1 you get 2.4RP for 1gold maintenance. Perfect option to spend your leftover money at the end of turn.
Sybil: S1N1 early SC deterrent with some fortune telling (5) for 80gold, not sacred. 2 mapmove as all of your units (save one). Buy them if you need a guaranteed S1 caster in the next turn (arcane probing).
Gittite Commander: 27hp, 12MR and 2 precision would make him a decent thug chassis, if you'd not have several better options for that.

Abba: A giant N2+100%FES mage for 120gp. Decent stealth (10), heretic - the latter is rather an annoyance if you keep them in your own domain. You would as they are healers (30) to remove afflictions from your sacred troops, but it is managable. Unfortunately has no leadership to lead stealthy troops, but kit them up and send deep into the enemy provinces to raid weak PD. Recruit one early for manual site-searching and to cast Haruspex.
Yeddeoni: 1D1E+100%FWSD. 2E allows earth boots/dwarven hammer and Gnome lore, 2D allows you to make skullstaff and to expand upwards in Death magic. Recruit them at least until you have enough 2D and 2E mages for diversity. Thug and army buff, esp. with Earth boots.

Kohen: B1H2+100%FES for 150g. They have great uses. Recruit them early - better researchers than the sage, less upkeep than your other cheap mages, and latter you can use them. They benefit from bless, the F randoms are blood hunters, the E and S ones have self-bluffing capacity (Stone/Iron Skin or Body ethereal/luck/twist fate).

Seren: 200gp equipped sacred thug (in CBM cheaper), 42HP, 18 defence and 17 protection with magic sword, unfortunately 73GP so you cannot buy it in the first turn.

Kohen Gadols: B2H3+210%FES for 400g. Have one with each armies for Divine Bless. S2B2 ones can Call Lesser Horror to clean up indies and weak PD on their own, with blood empowerment the E ones can make blood stones, or just cast Earth buffs for the army. With some S and B boost you can call whatever horror you want. F2B2 ones can build up your devil armies. Two blood empowerment for 105 slaves to forge Brazen Vessel/Armor of Souls/Soul contract for freespawn devils. Also, a recruitable SC chassis.

Your Kohen Gadols and Serens eat population but it is unnoticable with some growth scales, and they don't cause unrest unlike their EA ancestors. You also have site searchers for everything save Air and Water. You need empowerment to build the astral boosters but you have Earth and Astral income.

You have only two national summons, unlike Hinnom and Ashdod. The Se'irim (B2/B3) are great and cheap sacred demon troops, 8 can be summoned for 33 slaves by any Kohen Gadols. The Shedim needs B3A1, thus empowerment if you find enough air gems, or a pretender built such. The only problem you need wasteland to summon them, so on some maps (or if you are unlucky) you should not base your strategy on them.

Pretender: You have anti-SC giants, curse-spamming sibyls for deterrence, H3 priests and sacred troops. Even a 200gold thug. You do not need much help in research either. So the obvious choice is an imprisoned pretender with some bless and good scales.
Scales: O3 is necessary to have gold for your temples. Slot is not recommended, production would let you recruit more Gibbors. Heat 1 is preferred, so choose H1 or H3. I'd pick Growth - you could choose G3 and have only O1, that would help later in the game immensely. In LA you'd like to get all the gems possible, so choose Luck not Misfortune. Magic-1 is a no-brainer with your cheap researchers. You can blood sacrifice and have H3 priests so medium-strength dominion should be fine.

Now depending on whatever chassis you choose, you can have 3 minor blesses and either a further bless or 2 picks in air and water. I'd pick N4E4, the rest is IMHO optional - whatever you want or the pretender chassis has for cheap. I won't go for major bless, as the Gibbor are capital-only, the Se'irim are not guaranteed, and the Levites themselves do not worth it.
Of course W9 (suggested by Baalz for Hinnom Se'irim strategy) benefits everyone. E9 is good for your casters and recruitables - but the Se'irim has no armor.

Midgame should be based on blood summons, late game on your tugs and kitted out Kohen Gadols, plus summonable SCs.

Some CBM notes
There are much more valid chassis choices, especially if you like multiple minor bless/rainbow pretenders - even on titans. I like Lord of Rebirth for his 2E2D gem income - you have no death gem income at start.
Your recruitable everywhere S-random Kohens can Call Lesser Horror for 1 slave, and it is only Blood 1. You also need some early blood hunting. Read more on the Horror Strategy in Baalz' Hinnom guide.
Seren cost 100gold and the same 73res. Recruit a few in the early turns and send them on the way with a priest/prophet to bless. Any random from 20%FES is a bonus.