With arbitrary generation rules they are surely not. This is a counter example on 4x4:

  | 1 | 1 | 1 | 1 |
  | 1 | 3 | 3 | 3 |
  | 1 | 3 | 2 | 2 | 
  | 1 | 3 | 2 | G |

Or

  | 2 | 2 | 2 | 2 |
  | 2 | 2 | 2 | 1 |
  | 2 | 2 | 2 | 2 | 
  | 2 | 1 | 2 | G |

This seems to be able to be understood as a reachability graph problem of some sort perhaps.

Edit: formatting

Worth noting, since it was absent in the rules, that it is unnecessary to touch all of the squares.

I did about 10 4x4s and 6x6s and they were all solvable.