In golf, how are a 'mulligan' and a 'gimme' related?

A 'gimme' is a shot that players agree can be count automatically without being played, most typically a very short putt that is considered unmissable. It is still recorded as a stroke.

A 'mulligan' is a shot that players agree can be replayed without any penalty.

The common theme I see is that an informal agreement must be reached between players for both. Neither is allowed in the formal rules of the game, but they are common in casual play.

https://en.wikipedia.org/wiki/Gimme_(golf)
https://en.wikipedia.org/wiki/Mulligan_(games)