It will stick to the glass just fine once it starts to grow. Like others have said if you have it on a rock that you can move, moving it right up to the glass will let it spread onto the glass. Once it attaches itself it will be fine, it's just a matter of holding it in place long enough for it to grab hold. If you give GSP a place to go, it most definitely will go there.
Gluing things is very easy. You need gel super glue. A small amount goes a long way. Put a dab on it out side of the water, then stick it where you want and hold it in place for about 30 seconds. Others have had success gluing GSP to glass, I just haven't.