Gaga is a trend setter. There's literally no logical way of arguing otherwise, and you'd be very hard pressed to find much of anything she's followed any of the other girls on or hasn't reinterpreted or blended in a way unique to gaga.
That said, in the past few years the only thing that has actually caught on is 'yaaaaaaaas' and that was started by a fan and not her. So maybe it would be better to say that in 2015 she just does her own thing rather than setting trends.
Trend-setter (see: Dark Horse ---> Urban pop - last minute remixes - naming your fanbase - multiple wardrobe changes at award shows - electro/bubblegum pop - saxophone solos in pop songs- creative lyric videos...)
oh my