What model? Factory adjustable sights?
I have bought about a dozen or so Glocks over the years and only one needed any sight adjustment at all.
Ignore the ignorant Glock haters; worthless drivel. Might want to go to GlockTalk.com and ask there. They will likely refer you to trying to get used to the trigger pull or grip. There is a chart.
The trigger pull is 5.5 lbs. Same amount that my new 1911 came with. Only difference is that if I want to lighten the Glock trigger and smooth it out, it costs me $20 and 30 minutes with some polishing compound rather than $200 and months at a gunsmith.
A pistol that far off at 15 yards is either seriously screwed up, or the trigger puller is.