That might be the point, though. Just barely noticeable stuff here & there. Maybe a game is 5% cheaper with Game Pass than it is with Gold. Maybe you get an exclusive item in Fortnite if you have Game Pass, but not if you only have Gold. Anything they can do to have people realize that Game Pass is the way to go.
I don't mean to keep batting your ideas back (because I realise how annoying that is!) but both of these already exist...
Every game on Game Pass has a 10-20% discount to buy for subscribers (depending on how long it's been on the service) and that extends to all DLC/MTX too.
Game Pass Perks (a feature of Game Pass, as the name suggests!) regularly give free items and stuff away. We got free stuff for Smite, Forza Horizon 4, Warframe, PSO2 etc recently with it. It's a good perk! Spellbreak is giving their whole Chapter 1 battle pass away to Game Pass Ultimate subs at the moment.
I get your point though; added value to make people want to sub to GP over just Gold. Maybe part of that is marketing these perks better.