You need a few tools. It may be it broken off, in which case an easy out, with a left hand twist may get the remaining part out. Otherwise you may have to carefully drill it out by not wrecking the threads of the casting. Drill size for the case of drilling a hole before you tap the threads. Assuming that works, then re-tap the fitting. If you wrecked those threads drill it out for the next larger available nipple (Zerk) and tap in new threads. Look at where it sits and see why it broke. The new one may have to be a different design. Many types of Zerks are available. I suspect all threads are metric also. The taps may need to be those that can bottom out in a hole. Most taps are tapered and need space beyond the place for the threads for that taper.
There are some rules to think about for these locations for greasing. If you only grease now and then, grit can get in the joint and assist, along with the grease, to wear the joint. If you grease frequently you tend to keep that git out. If you don't grease at all, there is wear, but not as bad as if you are greasing only rarely . The grease and the grit combination make a good abrasive, better than the grit alone.