I just put together a list of all the topics I think could conceivably show up on a GRE Math Subject Test (without giving away any actual questions of course). It's a bit overkill, but I think that's a good thing — the more prepared you are, the better! I've deliberately marked things I would consider "stretch topics" with an asterisk (*) — take those with a grain of salt and perhaps start to focus on them only if you really feel confident in the rest.
https://www.mathsub.com/wp-content/uplo ... c-list.pdf
Hope this helps!
