I love
Gideon but
Ernest is irredeemably nerdy to me.
I do think
Gideon will at least occasionally be shortened to Gid. I've known people who insist on shortening the names of everyone they know, so I don't think it's possible to completely avoid it. It wouldn't necessarily be done by everyone all of the time though.
I don't think it likely that
Gideon will ever be shortened to Giddy.