GSPs will grow on anything and everything - rock, glass, they'll overtake other corals. I love GSPs, the way they look, but they spread like wildflowers so make sure that's what you're after.
If you want them on the glass, perhaps you could secure them there with 2 of those suction-cup clip feeders that are used for feeding nori or something. It'll take a few weeks for them to attach, but once they do you should be good to go.
You could also use a few small drops of superglue gel.
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