Lot of differences between the 265 and gulfstream, cockpit size is misleading. Gulfstream has a bracket putting engines further off boat causing you to reach far around them. If it has the aft bench seat, its even further, and the fish boxes in the transom don't help for fishing for a hardcore fishermen. The engines on the 265 are close to the cockpit, as close as you can get them. Don't worry about the notched transom and the fold down well on the 265, I've backed mine doiwn hard chasing a big bluefin tuna and the cockpit was dry and nothing came in even though we sure had it splashing hard against the well wall. The 265 has more cabin and more helm space. If you cruise with a bigger crew out the the greounds, the 265 will be more comfortable giving you a bigger helm and hardtop/enclosed area to sit in. Disadvantage is the deletion of the walkaround making walking to the bow tougher, but not bad. If you only go forward for dock lines or an emergency this is not an issue. You could fish off the bow of the gulfstream if you wanted, but I've owned walkarounds from grady and you will only do this honestly in calm conditions, they are no fun when seas kick up. The 265 and gulfstream both had aft facing seats for watching baits or for holding rods while fishing. The gulfstream has a transom seat on newer models, but is worthless when fishing, its faced the wrong way so its only good when running, but being so far aft and having a skinnier helm area and enclosure, means with any wind you will be wet there. It takes a lot of wind to get the cockpit of the 265 wet. Both are great riding boats and will eat chop well. The 265 will burn more fuel, but weighs more so in nasty seas is will handle better. If efficiency is more important a gulfstream may be better but you sacrifice comfort in nastier seas. If you want range, both are god but the 265 will run further. My 265 has a lot of range w/ F225's. I've run her 113nm one way, trolled for 8 hours, overnighted for tuna and then ran back 115nm to port on 198gal of fuel. For thr trip we were around 1.5nmpg...note I ran at optimum cruise 3/4 of the trip in and out, and at 5200rpm for 1/4 of the trip which is way above cruise to avoid a storm so brun would have been about .1nmpg better for the overall trip if we were able to stay at cruise.
You really cannot go wrong with either model, just depends what suits your needs better as well as your wallet. My Dad and I always wanted a gulfstream, but when we moved up we jumped over it and went to the 265 and have never looked back. The 265 is way more boat for the money honestly, but if you do not need all that then it may not be for you.