Well, I can't say I have definitely gained anything from pumping as I do other things besides just pumping, so I can't say scientifically I have gained from pumping, however, I'm pretty sure it has helped me gain what I have.
If you are talking of immediate results from overpumping, then yes, I have "gained" some. Back in the day I did an extreme clamping and pumping session which blew my length up to 8" (back when I was 7.5") and girth over 7".
Geez, I wish I would have taken a pic. That sucker was MASSIVE! The biggest downside is that it was squishy. However, it didn't have that donut and stuff like most people get, probably because I wasn't doing any extreme pumping, only extreme clamping.
The routine I used that day (which I have never even tried to repeat, I think it's so bad for the penis) was 3 [words=https://officialhydromaxpump.com/?uid=6&oid=2&affid=98 ]x 30[/words] minutes of clamping, no breaks, along with an hour of pumping 6 x 10 minutes at up to 5" hg. I'm surprised I could even get a hardon after that.
I did one extreme pumping session with water though. I must've used upwards to 10" hg, with one of the sets going to 30 minutes. I achieved probably a 7" girth that time, however, I couldn't really get hard and it was all lymph fluid.
I do NOT recommend doing this if you want to gain.