This gully is steep enough to elicit a Muscular Endurance training effect. The fact that your legs are the limiter (even with a HR=125) is the key observation. You are operating right at the endurance limit of those legs right now. The steepness is requiring you brain to recruit some faster twitch fibers that do not have the Aerobic capacity of their slow twitch neighbors who alone are not strong enough to supply the needed contractile force to lift your butt up the gully. So while HR is in what would normally be considered Z2 the effect is if you were in Z3-4. What you are doing qualifies as a ME workout as described in articles and a video on this site which no doubt you have watched ;-).
Your trail Z2 hikes are not having the same training effect because you don’t need to recruit so many FT fibers to hike at 130 on moderate grades allowing you to hike faster causing a higher HR.
Since these gully hikes are actually ME workouts I’d suggest not doing them at this intensity more than 1x/10-14 days and being sure to maintain plenty of less steep aerobic work between.
HR is an imperfect metric for intensity and training effect so you can not rely upon it blindly.
I hope this helps,