By the way, this looks like as good a place to ask as any, and I've been wondering.
What is the failure mechanism from a too-dirty gun? Do the rounds not feed anymore due to a dirty chamber or feed ramp? Does the slide not cycle correctly due to gunk in the rails? I've been curious. Say you had a lot of rounds through the gun and didn't have any cleaning supplies on hand (except a dry rag), but you needed to do something right on the spot to help reduce the odds of a malfunction. What would you do?
Mostly, my experience is failure to chamber. Doesn't seem to matter WHAT gun, rifle or handgun. I've shot a few firearms to point of failure, and it always seems to be failure to feed. The H&K G3 (and most other H&K's) will fail to chamber when it's real dirty. And they get REAL damn dirty, due to the fluted chamber. I run those really wet now, and it's not unheard of to see me open the car hood to take oil right off my dipstick and drip on the bolt if I run out of CLP. (When I had my old pickup that ate more oil than gas, I'd just pour it right from the quarts of reserve oil I kept in the back seat.)
The only exception I've seen is the AR 15 - you don't get the gas tube receiving key cleaned on top of the bolt you can get carbon lock and it can fail to extract or extract partially and give you the dreaded double to clear.
Back to Glocks.
When Glocks get REAL dirty, and I mean REALLY dirty, the kind of dirty that'd make most guys cringe and yell and say you're abusing them, they'll start to run sluggish. The slide will lose some momentum - rounds might not chamber all the way, and you'll find that you have to tap the back of the slide to get them fully in to battery. I haven't had that happen in a long time due to grime, but those goofy wadcutters I shoot occasionally cause it all on their own so it keeps me on my toes (those rounds, BTW, won't even CHAMBER in an XD, and give my 1911 all sorts of fits).
When it starts to run sluggish you really don't NEED to clean it, per-se, just take the slide off and spray some CLP on it. It'll be running (and smoking) just fine in no time.
Watch for the mag-well tho, that gets dirty run a rag through it or you'll have problems with mags not dropping free. That'll slow you down. A lot.
Good to drill with for awhile though, before you fix it. Keeps you on your toes to go from smooth reloading to stubborn reloading.