Sorry, I didn’t notice your post before.
You should be getting syntax coloring for *.gdshader files, as long as they’re associated with this extension in VSCode settings (should be by default). Even if they’re associated with godot-tools instead, that should still give you syntax coloring because it seems they also have the implementation for this, at least in the latest version.
If you’re not getting even the coloring, you can open an issue, but include as much info as possible so we can figure out why.
Now, if you’re talking about diagnostics (error squiggles) for lexical and syntax errors, then that’s available in early access, only to supporters who donated.
Note that this will still only give you some of the most basic error checking; most other errors and other language features you’d expect (e.g. code completion, hovers, API docs, etc.) are not implemented yet. I’d like to implement this, but I have no estimation for when this might be done, as it requires a VERY significant time of focused work to do, and I’ve been too busy with other stuff at the moment.