The above are correct. Glock says lead is a no-no. You are not supposed to shoot lead out of any polygonal rifled barrel (Glock, HK, etc.) that I'm aware of because they lead quickly, which leads to higher pressures, which leads to (possible) unpleasant explosions. I'm sure someone can offer a more technical explanation, but a traditionally rifled barrel with grooves gives the lead someplace to collect. That being said, if you shoot the heck out of a traditionally rifled barrel without cleaning, you can have the same leading issues.
That all being said, as long as you clean your barrel regularly and frequently check for leading, you should be able to shoot lead with no ill affects. You may have to use something stronger than Hoppe's 9 and brush the heck out of it, but it depends on the severity of leading. A friend reloads 9mm lead bullets to low velocities and shoots them out of his glocks all the time with no leading issues (I've shot them out of mine and haven't had any leading issues either).