If you can't get an advance, GC walk up tickets are usually much cheaper than EC.
Not so much any more. If you're travelling on a weekend unless you really want to save I wouldn't suggest buying a GC Only ticket. Their Off-Peak Return (SVR) from York to London is £94.10 whilst the Any Permitted Super Off-Peak Return (SSR) is £103.30. At a weekend if you pay £9 more you can travel on any train (including the GC services) between York and London.
Even on a weekday I'm not sure it's as good as once it was. The Any Permitted SSR is valid on any train due to arrive in Kings Cross at or after 1117. The GC Only SVR is valid on any southbound GC service other than the 0820 from York but this is the only southbound train of theirs which arrives before 1117 so you might as well buy the Any Permitted SSR.
On the return leg the Any Permitted SSR bars travel between 1500 and 1859 (1815 on Fridays). The GC Only SVR on the return portion allows travel on any northbound GC service apart from the 1650 but that is their only service which is in the restricted time for the Any Permitted SSR. So if you're able to restrict yourself to not being able to travel on the 'peak' GC services you might as well by an Any Permitted SSR as that will let you travel on any service at a valid time!
I'm sure for people who are just jumping on their services it makes sense for GC to have GC only tickets to make sure they got the maximum revenue possible. But if you're buying at the station or in advance of travel I'm not sure why you'd ever buy a GC Only ticket on the York - London route when for £9 more you can have more flexibility!