On a 2012 you should have them all at least removed and have anti-seize applied.
For me (a 2005) its too late... GM used a bad design with the glow plug extending through a long galvanized tube with the threads into the head on one end and the hex stamping on the other. The head is aluminum so galvanic welding sets in almost immediately and after a few years, they are seized. Almost impossible to get a seized one out without twisting off the tube. Odds are it will take removing the heads to have the broken bit removed by a machine shop.
I know this because I had two (one on each bank) fail. I never noticed because it still started fine - until the entire module went. When replacing the module, they found the two bad plugs and so I said to replace them. The first one broke off as above but didn't leak and the mechanic didn't want to touch the second one because he probably wouldn't be so lucky. Would have cost $2500 to R&R one head and an extra $1000 to do the second at the same time. They said I still had 6 good ones so it would start (run a little rough for the first 15 or so seconds) and they're right. If I use the block heater for a couple hours, even at zero Fahrenheit it doesn't even use the glow plugs (use is dependent on coolant temperature). Now that I'm retired and the truck doesn't leave the driveway but for a few times during the winter, I can live with that trade.
Long story short... service them NOW or live with them when they fail (unless you have deep pockets).
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam