First time buying in gog store.
Is this game drm free how do I know if a game is drm free on gog?

Welcome.

All games on GOG are DRM-free. See the third paragraph of FAQ: What is GOG?

A few games have content or features that unexpectedly require an online connection or even third-party account registration. There's a list of them in the thread DRM on GOG: list of single-player games with DRM. There's no mention of XCOM so far.

Yes. I play this game on a laptop that is never connected to the internet.