I'm using it in Sweden as well. I wrote a Python script to populate the accounts from a list of accounts I received from my auditor/tax consultant, as such I don't know exactly what flavour of BAS it is.

I use a letter prefix for my transaction sequence numbers and GnuCash doesn't seem to understand it or put any value in it at all. This is definitely a weak point in my eyes and I'd love to have GnuCash support categories for transactions and strict sequence numbers.