[Kheldron]

Yes, there can only be 5 GE at the same time. If you cast one it will try to override one of the 5 existing GEs, selected at random.
If you read the manual, it's quite equivalent to casting a dispel, it depends a lot on the lvl of the caster and the number of gems you put in it in excess of its base cost. If the caster lvl and gems put in the spell you try to dispel exceed yours...well, unless you're really lucky and win an improbable RNG, you're screwed.