I have the GoGuns gas pedal on my carry Beretta 92X full size. It is the one that replaces the takedown lever and is of steel material. Its solid. Just as solid of a fit as the original takedown lever. Makes a difference for me and additionally works great as an indexing point. As far as holsters go, it was easy to modify my carry holster for it to fit. I took a heat gun to the holster, shoved the pistol in, press fitted the area around the gas pedal, and quickly cooled it. Took all of about 10 minutes to complete. This may be partial to the Beretta 92, but it does not add much noticeable bulk. From the side of the frame, it just sits ever so slightly taller than the slide release. It was a good buy for me and have it on me everyday.
I am not a fan of the rail mounted ones, but I have also never tried one that installed that way. The Beretta 92 has the added benefit of a locking take down lever which likely adds to the security and rigidity of the gas pedal when applying pressure to it. I would be curious to see the fit and feel when applying pressure to say one for a P320 which does not have a locking takedown lever or one mounted to the rail.
Antimatter Industries has the "Wing" which is a gas pedal that is spring-loaded and unfolds into position once the pistol leaves the holster and folds back up when reinserting. Think a cut down AR15 spring-loaded dust cover mounted to the side of a pistol. I believe the part is actually a replacement locking plate for Surefire and Streamlight pistol lights so it requires the light in order to mount. Again, like above, not my cup of tea, though an interesting approach. It however appears to be gas pedal that you leverage downward and not inward like GoGuns.
https://www.antimatterindustries.com/wing-buy-now