Everything you've done should work. Just check that your gas coloniser design isn't quipped with a rock colony component. (I've made that mistake before now)
The only other possibility (the most probable one) is that you clicked "only colonise own planet type" during game setup. If that's the case, there's nothing much you can do about it until you start another game.
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SE4 Code:
L Gd! $ Fr- C- Sd T!+ Sf-- Tcp-- A% M>M+ MpD Pw RV Fq+ Nd- Rp+ G-
/SE4 Code
http://www.sandman43.fsnet.co.uk/se4main.htm