...how? Gaga really did have a place in popularizing electropop. Britney did as well, but it's not as if one or even two artists alone are responsible. A lot of artists contributed like Gaga, Britney, Nelly, Rihanna, and BEP.
But, as Eric. just quoted, ElectroPop wasn't that big in Music, R&B and Hip Hop were.
Trends are always "set" way before they are big. That's why it's called trendsetting. The majority of America's dance music trends were set in the UK and brought over by an artist or a producer years before it made it big. Once it's brought over, it spreads all over until it makes until it's flooding the scene.
Just because an artist does it right when it really starts to blow up doesn't make them a trendsetter. If anything, it makes them a follower.