I find that if they dont have lots of bamboo or sticks to perch on, than they rarely ever use anything else...giant day geckos are big geckos, and if theyre not given a strong or stable perching spots, they will stay on the glass. This may not be a big problem now, but geckos that have spent all theyre life sticking to glass, the force of gravity is too strong for theyre fragile little tails, and it gets jerked to the side. I beleive This is called 'flap tail'. So, try getting it some dry bamboo or thick branches for them, so you dont encounter anyt problems in the future...
M.N