If *you* write some software, on your own, without using any other sources, then *you* decide exactly what the license will be. If you say it can be used with Photoshop, then so it is.

However, GPL plugins can't be used with Photoshop. So if you say that it is GPL, and at the same time you say that it can be used with Photoshop, some people may be confused as to what you're really trying to say here. But since it should be quite clear that you want it to be used with Photoshop, the only reasonable interpretation is that you mean that it is GPL with the exception that you allow use with the non-GPL Photoshop.