When I have both plugins activated (GB and WishlistMember, GB seems not to be active, and this message is shown: Your site doesn’t include support for the “generateblocks/container” block. You can leave this block intact, convert its content to a Custom HTML block, or remove it entirely.
After carrying out various tests, I concluded that Wishlist Member Plugin and GenerateBlocks are incompatible when Wishlist Member is installed on a multisite. However, if these two plugins are used on a single site, they work seamlessly together.
I’ve reported this issue to the WishlistMember plugin author, so now I am waiting for their answer.