They alternated each night on this tour.
I also saw them a couple of weeks ago with Mastodon closing, but I honestly think it would have worked better the other way around. It felt like the crowd used up all of their energy on Gojira and didn't have much left for Mastodon. I saw quite a few people leave early. For what it's worth, I was there mainly to see Mastodon, but Gojira ended up being the highlight for me. They went hard.
One of my favorite albums to come out in the last few years is Etemen Ænka by Dvne. The whole thing is great, but I'll give special mention to the riff at the end of the song Court of the Matriarch, which is so heavy it'll make you feel like you could stomp the earth in half.