There are several reports requesting this feature:
Forced Builtins (http://pydev.org/manual_101_interpreter.html#PyDevInterpreterConfiguration-ForcedBuiltins) doesn't work for me adding gi, gi.repository or even adding external libraries in PYTHONPATH with /usr/lib/gireporsitory-1.0 or /usr/lib/python3/dist-packages/gi
This should be done by default almost for gi repository, because is very common have source depencies with GTK, Unity or others in Ubuntu when you write some code.
So please don't mark this as invalid or works for me and let's trait to solve in easy way for users. Users wan't see warning red lines or add messing lines like @UnresolvedImport in his code. They want a robust module autodetection.
If this can be done in terminal:
>>> import gi
<module 'gi' from '/usr/lib/python3/dist-packages/gi/__init__.py'>
PyDev should autodetect that gi module is installed and present.