I don't understand what the point of trying to get a higher gamerscore, it's not like you get anything for it (except maybe bragging rights), you just waste time trying to do things like collect ridiculous amounts of collectibles or perform repetitive actions in an effort to get a higher number, seems like they only do it to make people keep playing or replay games that have no other replay value. Ranting aside you'll just have to do the quest again and make the other choice unless you have a save from before you did that part.
Well I don't try to get most of those stupid achievements out there, such as collecting certain items or w/e. But
with Oblivion it is actually very easy to get all the achievements, you just have to complete the game and
each faction so you are not doing anything extra in order to get max GS for the game.