I guess it depends on what your definition of "gentle" is. You seem to fall in the "gentle" = "easy listening" camp, which doesn't line up with my definition. To me the production is the biggest factor, and Howls of Ebb are very, very light on distortion with a very airy and expensive master. They are surreal, yes, but not very heavy or aggressive. It's like wandering into a perfume store.

But other people in this thread are using "gentle" to describe anything that's compositionally simple, which misses the entire point to me. Like, Death and Obituary I can see being gentle (mostly because of the production, not necessarily because they have simple riffs and song structures), but Bolt Thrower? Jungle Rot? These bands both sound like a cinder block to the fucking face, nothing about them is "gentle" in the way that I'd use the word.

death, cynic, and atheist are all decent contenders, though how primarily death metal you want will of course come in to play

on a totally not-what-you-meant tangent, i'll throw in lykathea aflame - they're solidly in the brutal end of the spectrum, but the term gentle definitely also applies to them more so than any other death metal band i can think of (mild or tame, definitely not)

This is a question phrased in such a way that every possible answer sounds idiotic. One of the primary attributes of death metal is that it isn't gentle, so to answer it you need to stretch either the definition of "gentle" or the definition of "death metal" well past the breaking point. It's like asking which barbershop quartet is the most brutal.