Glock says not to reload
Glock says not to reload, as it will void your warranty. The reason is that Glock chambers are not 100% supported and may/will bulge cases slightly in one spot. To understand this take yours apart and look into the breach end of the barrel, slide a cartridge in. You will see that the "ramp" area does not support a small part of the case. Different generations of Glocks have different degrees of this issue. Newer are better; some one posted pictures here.
There are products on the market to "repair" Glock fired brass, essentially by pushing the case all the way through a die. I am aware of these but, have not experienced the need to spend money.
Bottom line is it is a personal decision. I do reload for my 23. I use once fired 40SW brass (don't know if it was fired in a Glock before). I shoot target loads and have never had an issue or noticed the supposed bulging. I have reloaded the same cases again, with no problem.
Another possibility is using a different barrel. I use a Lone Wolf 9mm conversion barrel, which is fully supported, stainless steel and conventionally rifled. Glocks come with polygon bore rifling, and Glock says not to use cast bullets.