Does the Gmusicbrowser has option to add (or at least read) custom tags?

not yet, but it's almost done in v1.1.x, it has been so for a while already actually :( but it will come.

Though if you don't want to wait, you can tell me what tags you want, and I can give you the few lines that must be added. The lines may change a little with the future v1.1.6

I would like to read tag "rating". Since gmb cant store that info in file I started to using foobar to rate files in a comfortable way. If you add option to read will I be able to edit it?