Just in case you're still wondering, "GG" stands for Galarian Gallery in this context. It's like an additional mini set within the expansion. Not all expansions have them, but some do and as far as I know they usually consist of 30 cards. So in this case an expansion will have it's base set, then secret rares that are numbered above the base set numbers, then 30 Galarian Gallery cards. Some expansions have Trainer Gallery cards instead. Those would be marked as "TG".
Just in case you're still wondering, "GG" stands for Galarian Gallery in this context. It's like an additional mini set within the expansion. Not all expansions have them, but some do and as far as I know they usually consist of 30 cards. So in this case an expansion will have it's base set, then secret rares that are numbered above the base set numbers, then 30 Galarian Gallery cards. Some expansions have Trainer Gallery cards instead. Those would be marked as "TG".
Oh cool, now I know! Thank you for sharing with me.