From the GCD readme:
GCD will work with Wakim’s Game Improvements or similar mods which make changes to the speed of increase of skills by altering the game settings (though clearly BBB might conflict with any other birthsign mod). These alterations use a different system from mine, and will cause no problems – in fact, since I make no changes (on average) to the rates, mods which make sensible changes to the increase rates should complement GCD well.
Both Better Balanced Birthsigns and WGI change some birthsign stuff, so check the readmes for both and decide for yourself how well they mesh up. BBB changes very little and if I remember correctly I used both changes to my satisfaction for over a year during my last play through.
I always had GCD loaded first in my overall mod list because I didn't like it bumped around in the load order when I added mods.