I played the PS2 version ages ago, and I mostly enjoyed it aside from some fiddly controls (duh) and the fact that so many of the missions had complicated goals, when all I really wanted to do was, well, scare some fools and defeat some excorcists.

So, does the GOG version have any kind of free/skirmish mode in which you just have to scare people away, or is it just the missions?

It's just the missions.

I usually just load up one of the hospital scenarios (mostly "Phantom of the Operating Room") or the military base if I just want to scare people (or drive them mad). The objectives on those aren't hard and you don't need to free any spirits to win. Plus, both have a ton of mortals to terrify :)

Seconded. I've actually only succeeded once in driving every mortal insane in the army base. One of the female Privates (or perhaps she's a Sergeant?) in particular always gave me trouble. Wandering around, utterly alone in a now abandoned army base, diligently keeping up her patrol and an absolute bastion of sanity against all the horror I could throw at her.

I don't actually recommend doing this for points, incidentally - this took me an hour or so to manage, zero pumpkins...