Also, may I point out that making a previously straight character gay is no longer revolutionary, and probably never was advisable, save in rare circumstances. Especially when it's easy to simply create another character with any characteristics you want.
Back to the main question though, PG would be well suited to a movie because as I said she's considerably less defined as a character than say SG, and thus easier to fit into any sort of story the writer desires. She'd work in a straight superhero movie, a sci-fi movie, a horror movie that subverts expectations again, as did Buffy, (fitting perhaps as Buffy was one of the only shows that ever mentioned Power Girl, with 'Lil Buffy playing at being her), a light action or sci-fi comedy, heavy drama, even in a fantasy setting, or any combination of the preceding. Whereas Supergirl is trapped by what audiences would expect from a movie starring her. She's trapped by Benoist and before her all the animated versions and by Helen Slater's version. Most audiences would have no idea who PG is, or what to expect from her, beyond whatever marketing would tell them about her.