Hi, I used GCD up until lvl 17 but now I changed to Madd leveler. So I put the madd esps and removed GCD's, used the console command and it said madd leveler was initiated. But I have a mod (don't remember which one) that allows to speak to people and ask them to be a companion. And when I do it, to one of these NPCs, it says I should be level 15 or higher, while I'm lvl 17! Even the stats say I'm lvl 17, maybe removing GCD screwed this up?

Make a back up of your saved game, and try typing "Player->SetLevel 17" in the console. And take note of your attribute and skill levels before and after you do this to see if they are altered.

EDIT: BTW, i'm thinking of using madd leveler, But I don't like the way it handles level uncapping (through fortify spells). Is the level cap not handled by a gmst? If it is, could the cap be raised by altering a gmst setting?

No, it's not, unfortunately. Elys was forced to use an OBSE + DLL combination to make it work in Oblivion.