Advantages (or downsides) of GVim over Vim to edit code
In gVim you can select the font, vim depends on the font the terminal provides. And it's the same for colour support. Gvim has full support, vim depends on the terminal.
Gvim additionally has menus and a toolbar, which vim lacks.
One big advantage of vim is that, since it's a terminal application, you have a full fledged terminal at your fingertips. gVim has very rudimentary terminal support. This is handy if you run
:make, for instance.
Install both and try them both, they use the same configuration etc. so there's no risk.
As far as differences are concerned: one is a graphical tool one is a command line tool. So the advantage of vim to gvim is that you can use it easily over an ssh connection. (You can do the same with gvim by tunneling X but that has quite some overhead.
gvim package shall have all vim coming along. Meaning, if you wish to fall back to vanilla vim you can.
Anyway, the biggest difference IMO is that vim depends on the terminal to handle special characters and key-sequences. If you wish to map and exploit things like
<m-ins>, ... just forget about it with vim.
Of course gvim has menus, mouse, toolbar, etc. But that's just a small bonus compared to a fully functional keyboard.