In my opinion Gaia's should have an ability, and that is +5% artifact production (maybe that is a bit high, if so maybe 2-3%), see u get 5 gaia seeds but u also get 5 mass surface melters both gaia 22x and mega 22x oceans can produce the same amount of artifact production with the exact same structures on them, but see u would in reality get more artifacts from a gaia then u would from an oceanic considering on a gaia u have water and terrain, other things like metals ect..

The ability appears once a gaia seed is used

Nope you get a ability for one of them you have to think of an ability for all of them to balance it out, so if you can think of ones that make sense and are balanced then it could be a good idea.

Only if they evolved civilizations to build these artifacts. They are not a natural resource like minerals are so that makes no sense.

-1 anyway. Unbalanced. If gaias had this ability they would need to be much more difficult to obtain and I doubt an already existing mission will be modified.