Hi Matias,
When a key is bound to a menu you first need to clear the menu binding
and this can only be done inside .gvimrc (_not_ .vimrc ... uh, at
least I don't think .vimrc will work). See ":h :macm" on how this is
done. Something like this should work (not tested):
macm Window.Select\ Next\ Tab key=nop
nmap <D-}> :bnext<CR>
macm Window.Select\ Previous\ Tab key=nop
nmap <D-{> :bprev<CR>
Björn