It's very easy to translate the GUI using poEdit.

poEdit installation

Ubuntu

sudo apt-get install poedit

Download poEdit 1.5.2 and install it.

Download gimx.pot.

Start poEdit, and click on "File > New Catalog from POT File...".

Select gimx.pot, fill the fields (don't edit the other tabs), validate, and save as <language_code>.po.

To see what your language code is, check this page.

Example for the French language: fr.po.

Translate the messages that are not translated, and save.

mkdir -p /usr/share/locale/<language_code>/LC_MESSAGES
msgfmt -o /usr/share/locale/<language_code>/LC_MESSAGES/gimx.mo <language_code>.po

Generate gimx.mo with poEdit, and put it in the GIMX installation directory, in a share/locale/<language_code>/LC_MESSAGES directory.

Start gimx-serial, gimx-bluetooth, gimx-fpsconfig or gimx-config, the GUI should be translated.