I got a little curious and decided to make a new save to try and do what you did to see if it would appear on my end and almost literally 10 seconds after I chose tease him back he showed up again and let me change my choice from tease him back to encourage or reject the offer.
also I locked in everything you said you had locked so I don't think it's that.
and a quick question, do you restart from the literal beginning or are you loading previous saves from before you made that choice? and what days do you look for him? I think I remember reading somewhere you can't find Gunnar in the exact same spot after unlocking Northcrest, or maybe it's meeting scar and winning. also I decided to check if I could get the scene on a save where I had already unlocked Northcrest and even when he was near in bareshade he wouldn't come out on the tracks after checking for a couple in game days he was around and days he wasn't.
and another question, you said you can encounter him and the merchant at the river, does Gunnar have an option when you talk to him to ask for a rematch? if it does, you can actually use that by seducing him and submitting early which will unlock other scenes by increasing his like, not the track one though. I believe that one might get locked after a certain point of the northcrest storyline. not sure where though.