Hi there AirCav, and welcome to OFPEC!
First of all: are you sure the unit inside the vehicle is "units A_8Crew select 1"? Otherwise you'll try to feed a nil unit into the code, which will always cause problems. I do get what you're trying to do though - always select the unit second in command to the leader to replace the dead gunner, so there should be no problem. Make sure A_8GNR is properly assigned to a unit by adding a "hint format ["%1", A_8GNR]" after assigning the variable - if it returns the typical "scalar bool array string 0xef...." error, then it means that gunner doesn't exist and everything you do will cause errors.
Oh, and make sure the M119 is named correctly too!
Secondly, you're not using the moveToGunner command correctly: as far as I know, the moveTo commands, new to ArmA, are equivalents of the actions you get while sitting inside a vehicle: i.e. "move to gunner seat", "move to driver's seat" etc. So it only works while the unit is actually inside the vehicle. Which might be causing the errors.
As to the actual complicated matter of getting your guy unassigned from his current vehicle, then assigned to a new and ordered to get in...well, it's more complicated than it seems
If the M119 site isn't central to the player (i.e., he can't really see it most of the time) then I suggest simply using a moveIngunner command to teleport the poor guy, since that's just plain the easiest solution. See, AI squad leaders are twitchy with the whole ordering people into and out of vehicles - ever tried as a grunt enter a vehicle without your leader ordering you? First he tells you to exit, and a moment later he tells you to board it again...they get confused easily.
It would be easier already if the man wasn't inside another vehicle to begin with - why not have him loafing somewhere nearby? Stop him using either one of the disableAI commands (such as "Move") or doStop (which will sadly be disabled by a save/load, I think). Then just enableAI/dofollow him before assigning him his new spot and ordering him to enter the gun. If you absolutely have to have him inside the vehicle, I would suggest using the "eject" action instead, together with an unassignvehicle command, potentially a ordergetin false. And only THEN reassigning and reordering him.
Finally, although this has nothing to do with the error message, you might want to make sure the leader knows the gunner is dead before going anywher else. Sometimes they forget to check, which will most certainly screw up any "ordergetins" for the gunner position. Simply add a reveal command somewhere in the beginning of the trigger between the dead gunner and the leader, should do it.
Note, in many cases the AI leader will be clever enough to independently order someone else to fill up the gunner seat, although it might as well be himself...
Good luck! It's a tricky business!
Wolfrug out.