Not wishing to steer things too off course, but.....
There are some products which contain > 90% GBL but which are used for completely unrelated applications to the usual uses found for GBL. My bet is that if these products were imported, unless they are on a banned list already, there would be no cause to check them further. I could be wrong, but I also still frequently hear about GHB being available, so it's coming from somewhere.
GBL is much more obtainable than GHB simply because it's used in so many diverse areas. Mind you, about a year ago I did hear a stranger at a party bragging that his mate had a warehouse full of 200L drums of GHB. His bottle certainly contained a salty, rather than chemical tasting clear liquid.
While I tend to agree that most G is 1,4, I do think it's being rather naive to think GHB isn't around at all. GBL can reportedly be made from GABA, available from most health stores. The product may not be totally clean, but several have testified that it works. And if they ever ban GABA, there's always the more difficult route from scratch using butane.