I am considering buying this. is it possible to compete against Steam players?

Yes 100%

They really need to add Crossplay to info card on each game's page.

That wouldn't apply to this game anyway since crossplay is about connecting modern games which use Steamworks and Galaxy while Impossible Creatures uses neither since it was released all the way back in 2003 and, unlike the Dawn of War games, wasn't updated to use these newer multiplayer APIs.

The reason that the GOG version is compatible with the Steam version is simply that Impossible Creatures apparently uses its own lobby system which is hosted by Relic Entertainment (and is completely beyond GOG's control). Additionally, just like most old multiplayer titles on GOG, the game allows to host custom sessions and connect directly by entering an IP so the game will have working online multiplayer even once Relic, Steam and GOG are long dead and gone.

That said, GOG admittedly totally could and should add some more tags to specify the exact type of multiplayer included in its games.

This version of IC doesn't have anything to do with Relic. Nordic Games (THQ Nordic) owns the IP now and the new online service is being hosted by them.