Good to see it work, but coding large amounts of filetype specific functionality into Geany core is not the way to go.
Instead the code in core should query a plugin that plugin uses gocode to answer the question. Then another plugin could support [Language Server Protocol](https://github.com/Microsoft/language-server-protocol) and another could use [libclang](http://clang.llvm.org/docs/Tooling.html#libclang).
When the plugin is loaded it should register to be called instead of tagmangler for autocomplete. Being able to register for other functions can then be added as separate PRs, eg formatting support, highlighting support (Scintilla willing), syntax error display etc etc
So the first thing is to specify the design of the interface to the plugin, and KISS.