Massive Spoilers for the series:
Steins;Gate actually starts a little late & around episode 8, will start to get really depressing. However, 24 & 25 are the happy ending side.
S;G0 is the 2nd side of a what-if 23B didn't end as it did, However, it is massively necessary to watch due to it being a massive plot point in 24th episode of the OG S;G. This is also why many fans recommend to watch it all & then watch 24 & 25th (Egoistic Poriomania) episode of OG S;G at the end when you're familiar with it all. It packs a better punch as a story closure that way.
I just watched it normally due to being lazy, but you can do it the recommended way you have too (the one where you watch 24 & 25th episode after you watch everything, that way it packs the better punch at closure I'd say.
Also, the movie's canonicity has always been a bit vague, but I'd recommend watching it, it's good. However, do it before 23B.