Read the information section. It says how to care for a GBB and also how to fill it with gas.
I bet you are not filling it the right way.
You have the take the magazine and turn it Upside down, turn the gas can so the tube faces down too, and fill the magazine with the Liquid gas. You should get at least 2 full mags of shooting with one fill.
If you want better information, say exactly what kind/brand of Glock you have. Maybe one of the models is a problem.
Here's the freebie of the week;
http://www.airsoftcanada.com/showthread.php?t=21513