Actually, both Gudbrandsen and Gudbrandsson are "correct", so to speak....they are both patronymic version of "Gudbrand". The difference is just that the Icelandic version has two "s" and uses -son, while the Scandinavian (mostly Norwegian, I guess) has a single "s" and ends in -sen.

Anyhow, if you need any help tracking down the Icelandic "Gudbrandsson", just let me know....I'll look them up in the "Islendingabok" database, which covers practically all known Icelanders.