With pango 1.44 gvim with gtk gui will use too large character cells since pango now uses the global font ascent / descent / height / advance parameters. This merge request fixes the global font metrics to match the requirements named here With this change and that vim patch my gvim finaly uses the correct char cell size.
Since pango no longer uses the values in the EBDT table it seems to be necessary to put each font face into a separate font file. This should maybe be documented. See a related issue on pango.
To fix the vertical char cell size I needed to fix a rounding issue in vim, too. Pull Request