I really don't understand the need to classify every band into a specific subgenre (not necesarily aimed at you, I see this happen all the time); aren't the unclassifiable bands usually the most interesting..?
Like Terra Tenebrosa, to name but one not so random awesome band..?
In the case of Gojira, the death metal roots are pretty clear - especially on their earlier work. You wouldn't mistake what they do now for any other genre if Joe still growled. And if you look at their single biggest influence, things don't get much more death metal than Morbid Angel was. Gojira just progressed pretty fast to a point where they don't simply fit into the death metal genre. So I suppose that could make them 'progressive post death'..? Or if you take the subject matter into consideration, extreme eco metal? Green-core..? Technical tribal t(h)rash?!
They're definitely progressive-ish, but I can't think of any album of theirs that sounds especially death-y, not even going back to The Link. They're a highly Meshuggah-influenced band, like they're about as Meshuggah as it gets without the goofy polyrhythms (and that's a good thing, IMO). I don't hear ANY Morbid Angel in their sound.
I don't see what's wrong with analyzing sounds, personally. It helps figure out what we like about the music we like. If you just go "omg itz all metuhl" then you find yourself going "okay well what kind of sound do you like in your metal?" and there's no way to really explain. You can name BANDS, but that's all. Classification in music, like chemistry, helps us categorize. Nothing wrong with that!
As for Terra Tenebrosa (funny you brought them up... ) they're a post-metal band. There's some doomy/sludge in there and a few oddities, but I feel pretty confident in calling them post metal and being done with it.