Jeff, Co Steve explained it well. That gas pipe should not come out on it's own, but when held in by the gas bushing, they do have a little wiggle side to side and up and down.
This helps the op rod close over it, if it were rigid, the op-rod would have a hard time.
If your gas pipe pulls out, you are either missing the bushing, or the bushing is too short, thereby not doing it's job securing the gas pipe.
When you have the gas block separated into the two halves, to reassemble:
put the gas pipe into the lower gas block, and turn it so the bushing will nestle down into the recess in the pipe and hold it in.
Then put the lower block assembly up against the barrel so the top half of the gas block goes into the recess in the barrel.
You can see that barrel recess in this pic, and laying on the barrel, a .100" bushing from a Mini-30.
Then holding the lower block up against the barrel so it stays, put the top half of the block on and start the 4 gas block screws.
Tighten them to 30 inch pounds in a criss cross pattern, ensuring that the gaps are the same on each side, and the same front to back.
If you don't have a torque driver, get the screws snug, then turn an extra 1/8 turn.