They both write nostalgic rock and roll. Sure there are probably some Japandroids hipster fans who think they are too good for Gaslight, and plenty of Gaslight fans that dismiss Japandroids as hipster bullshit without actually listening to Celebration Rock. And of course, there are the people who legitimately don't like one band or the other. But they are lyrically similar enough and both have good music that I think there is a larger fanbase overlap than you're giving them credit for.
This, pretty much. Even if the fanbases don't overlap that much, I think that's the point. Kind of a win-win.