I guess Gilda couldn’t cut it in Gryffindor Academy.

I should point out that Gilda's most recent official (though not necessarily canonical) appearance was in the book Twilight Sparkle and the Crystal Heart Spell, which takes place between seasons 3 and 4. Not only has she not tried to better herself, but she actually turned Trixie back to evil and they're working together now. I suppose they deserve each other.