hi, to get Gunnar's cumrag you need to choose option in his dialog 'laundry' (when you built new houses in bareshade he's gonna constantly stay in one of them, you can ask him - if you have good relationship - about helping him and you'll get new option in dialog 'small tasks'), there when you gonna do laundry for the first time you'll be able to steal the cumrag :)
I had this scene with botLogan and Caleb few times, but I guess it's pretty random, maybe on a specific day, I just remember that it's always the evening in the Caleb's place