my friend is thinking of buying battlefield 5 on steam since its on sale today and I have it on GOG. can we play together even if we bought it on a different platforms like steam and GOG?

Are you sure you mean Battlefield 5? It's not on GOG (though that would be nice).

I don't think you have it on GoG. I think that it's just showing up in your "Galaxy 2.0" library as you have integrated your account with Origin or whatever other site you bought it from.

As it's a DRM'd game, Galaxy will launch whichever service you bought the game on. So - the crossplay will be between Steam and whoever you bought it from (not GoG).