Want the correct short answer?
It's perfectly OK to shoot either hard cast lead or swaged, (but hard) lead bullets out of ANYBODY's cold-rolled, mandrel-formed, polygonal-rifled pistol barrel. Whatever lead bullet you use needs to have a BHN (Brinell Hardness Number) of greater than 12 and less than 25. What does this mean in the, 'real world'? If you can easily scratch the lead bullet you're using with a knife blade, (or even your fingernail) then it's, probably, too soft to use in a polygonal barrel.
With a Glock it's always a good idea to slug the bore when you're determining what size lead bullet to use. Ideally, you don't want to be more than .001" over bore diameter; and you want to keep bullet velocity below 1,000 fps. A BHN of 24 is considered to be, 'universally acceptable'; but it might be, in fact, too hard for many Glock barrels. People who keep track of these things tell me that a BHN of 16 to 19 is often ideal.
Stay away from real soft, easily indented, lead bullets; carefully watch your polygonal barrel for excessive leading until AFTER you know how much a particular lead bullet is going to buildup; and you should be fine. I don't think I've ever gone more than, about, 150-175 fired rounds while using lead bullets before I stopped and scrubbed out the barrel. If I were using a new lead bullet I'd certainly check the bore every 25-50 rounds until after I know what it is doing.
I have shot many tens of thousands of Berry Manufacturing's plated (copper washed) swaged lead bullets through two of my 3rd generation G-21's. All without the slightest problem with leading. A large percentage of these bullets were, also, loaded in front of, 'healthy' powder charges. (I kept them well below 1,000 fps, though.)