Gorricks you can do whichever ability category you want, Masters is 30-39, vets is 40-49, grand vets is 50+. Age is taken on the 31st December of the year of the race, so if you're 30 this year you're a master.
Southerns are BC ranked, so it's slightly different in the ability cats, for expert or elite you need a BC licence in that respective category, you can do one round in sport (there's only one left!) but you won't get ranking points without a licence. You can do open.
Gorricks also have a super master category, which is a 4 lap masters race done by the quicker masters. Southerns don't have that, the masters race is still 4 laps.