Any games on Gog with that mode ?
Any custom maps / files ?

Thanks .

As in you can play against the computer/AI?

- Impossible Creatures

While you select maps from those already in the campaign, the game lets you create your own army of animal mutants you can pit against the AI.
It's a real-time strategy game.

- Battle Realms

Also a RTS. No custom files though.

- Dungeons and Dragons - Dragonshard

Another RTS.
I think the game only lets you play as the Undead in such a mode and not in the campaign.

- Heroes of Might and Magic 3-5

You can play against AI players and it's turn-based strategy.

- Age of Wonders 3

I'm certain of this one, but I don't know about other games in the series.
In AOW 3 you can even make your own heroes.
I'm not sure how this strategy subgenre if referred to though.

- American Conquest

A RTS game.
I haven't played it in ages, so I could be wrong.

- Disciples 2

Not much freedom since each map comes with an objective and a selection of playable races, it does have such a mode. It's somewhat similar to Heroes of Might and Magic in terms of gameplay.

P.S. Does "skirmish" only apply to RTS games?

I'll edit the post if I think of any others.

Unreal Gold has some awesome botmatches. You can skirmish 'till you pass out and no internet connection required!

https://www.gog.com/game/unreal_gold

Stronghold Crusader. You'll also be hard-pressed to run out of custom maps. ;) The only other one would be Shadow Magic, which features a map-generator for non-repeated skirmish play.

Total Annihilation

Not a gOg game, but the free Warzone 2100 also has it. www.wz2100.net