Gaga's first 2 years on the scene was huge because of the shock art and the eccentric outfits. Before she found fame, most musicians were pretty ordinary with a few exceptions (Madonna, Cher, Gwen).
Now it seems musicians have found courage to try to do the same as Gaga (or have they realised the outlandish outfits will just ensure headlines?)
The most notable people would be
Christina (The 2008 VMA's, Keeps Gettin' Better video, NMT)
Nicki Minaj (everything)
Rihanna (the Rated R era)
So was it Gaga that put the foot in the door for these artists to follow?
All the pop girls are influenced by each other. (I think Beyonce said something along those lines as well) Yes, they are influenced by Gaga, but that doesn't mean their influence can be coming from somewhere else.