Exiting the map

[Walrus]

By 'edge of the map' Weasel means a grey hex, not a terrain tile.
And yes, the unit will be gone the next turn, but be aware that your opponent will get the points for a kill of that unit. The game considers it to be killed, not saved.

I would never retreat a unit from the map, especially a tank with a missing main gun.
You can use that unit as bait, or cover for other full strength units. You can still use the MGs against infantry, or to draw fire from other armoured units. If it has good vision capabilities, you can use it as a spotter...or as transport if it has any carry capacity.

You can still provide a threat with the unit.
The enemy cannot see that you have no main gun, as far as they can tell, you have a fully functional tank on the map. They might waste resources trying to kill that unit and you may be able to draw units away from your actual point of operation.

Good luck.