I looks like this is a known issue, that Gravity forms and WP minify don't play nicely together.
And from this previous post (http://www.gravityhelp.com/forums/topic/error-gf_apply_rules-is-not-defined) it looks like the key to making them play nicely is to exclude the gravity forms scripts.
Now I know where I need to paste the script names (if that is right term) but not really sure what to paste.
The default ones seem to be things like :
jquery
jquery-core
If someone can please let me know what I would write that would be ace!