Can I watch Gintama' (2015) if I haven't watched previous seasons? I want to watch this new season but It will be my very first time with Gintama, so... Maybe should I start from the beginning?
Well, there's nothing stopping you from watching the newer episodes :)

Gintama is mainly episodic (as in, not much ongoing connection from episode to episode) so it's probably okay to do so. You might miss out a few references to past episodes though.

Of course, if you want to enjoy it fully I'd recommend you start from the very beginning.