Currently, gravity on planets has 3 differents possible states: high (very bad), average (nothing), and low (kinda bad). However, I think that it should be on scale instead (from 1 to 20+). 10g, which is roughly the gravity here on earth would be right in the middle, with no downside/benefits. Just to give an approximate idea (not fixing values), current high gravity would be between 15g and 20g while low gravity would probably be around 5g. Now here's the interesting part:
-Low gravity planets would have the upside to have their populations grow faster (much easier/less costly to land on those planets) but the downside of being limited when reaching high population (after all, a planet half the size of earth would have troubles hosting billions)
-On the other hand, high gravity ones would have the downside of growing slower (much more difficult to land, and less attractive to live there), but with the upside of not slowing down its growth even after reaching high population.
Basically, low gravity=fast profit, easier to start with while high gravity=much more powerful in the long game. Please note this is that for solid planets, and do not account for gas giants.
I don't think it should be really hard to implement aswell, since there's already something in the code that tell the planets what size they are on the map. Just gotta find this value, and use it to determine gravity.