Some other gab users I can follow, for this one there is error:
Sorry, we could not reach that address. Please make sure that the OStatus address is like firstname.lastname@example.org or http://example.net/nickname.
@lous, I'm sorry, I couldn't reproduce the error.
Before any further discussion, could you please try following that user again? A few things were introduced since this issue was opened, being remote-follow via ActivityPub one of them, so it is probable that this problem is solved by now.