I was looking for information about the matter since I have very recently encountered the same problem on Android.
Still getting problems on my end. No update available, tested many many ways, working nicely on Native, Gmail, Yahoo, and ... Outlook being the simplest one to code for, not doesn't display using the in-head CSS.
Maybe it has something to do with some META tag I messed up over time, but not quite sure...
Anyone has encountered the same problem ? More specificaly, I on my end use a Xiaomi Redmi Pro, Android built.
In the dream world, we might wish to hope that we would gather that many coders, having their very own webkit-ed design, for the effect to be felt by so many people, that Google / Gmail teams would react.
But I love the idea though :)
On the other matter of Gmail not evolving, and furthermore litteraly destroying enhanced Responsive workouts, it makes it so problematic... There are a great deal of HTML integrations that will fully regress to a normal display, and be a worldwide waste of time, which is a shame.
The non-support of CSS by Gmail is not new to anyone, especially them. The question is : Why in the world do they not do something about it ? Or even, create a whole lot of specific tags / class / -googlekit / who knows what, that us coders would use for integration nicely on Gmail.
If, for starters, they would let the style="display: ...", work in media queries, that would mean the world, and re-open a great deal of options.
So, see, I also dream on my end.