Gamma of the Film can be adjusted however you want, and is not specific to Film or developer. Water (e.g. city, distilled, …) used for developer is also in the game.
I did try with 1:50 and could not get Gamma 0.65 (density) even after 23 min developing (20 deg.C) so I did a try with 1:25 but Gamma did vary a lot along the curve again, so I gave up on HP5+.
Guys, I am not new with HP5+ but never in past had a such a problem.
I do not thing I will change Rodinal to any other developer for 400 iso Film. Buying another developer leads to need of another experience, complicates things and make me more overstocks. I will rather change the Film, and Tri-X is nothing behind HP5+ in any sense.
I do have around 5 HP5+ (36 exp each) from another batch and curiosity sake I will test and this batch too.
I see some guys ask why actually I spend time testing Films. I just want to know what I am doing, I know how to test Films, and I want to know how my Photographs will looks like before I take a camera in my hands. To test a Film takes me c.1 hour only and I have so many benefits out of that. I do not like "let it go, and whatever comes out sometimes fine sometimes who care, average is OK"..