Yeah, the design was a bit rubbish on that one, or the engine limitations were, anyway. The door is set just about where you'd expect it to be, right in the middle of the front of the house in the wall that's hardest to see or interact with. I suspect they were aware of the problem and positioned the two guards there to act as a marker, but that doesn't do much good when you can stand there scrabbling at the wall like a fool forever and not find it unless you're lucky. I normally end up feeling around for it with the pointer in examine mode, just like one usually had to do for the side door to the Skum Pitt in Junktown in the first game. Here, courtesy of
The Vault wiki, is a picture of the presidential mansion on proper display, which should help out a bit:
I don't often have reason to pay a visit to Carlson since most of my characters run to the squeaky-clean side of the karma scale (or at least too clean to go that far down the Bishop questline), but if I remember correctly, his kid will often be playing around out in the courtyard and you can send him in to tell his dad you're there, upon which he'll handily open the door for you and it's just a matter of you running in after him (though be quick, as I can't remember whether it's one of those doors that automatically closes itself).
As to Gauss weaponry, it
is all classified under small guns, and it is truly, truly top-shelf. Incredible stopping power, devastating crits, low AP costs... The sniper rifle isn't anywhere near the same league, nor are any other small guns with the possible exception of the H&K G11e (if burst fire is your thing). The only downsides are their rarity (and cost!) and the scarcity of their ammo. As J_Fred says, they're end-game guns, generally unavailable in static locations until you reach San Francisco, though if you're really good at surviving concentrated fire and you don't care about committing the odd mass murder here or there they can be looted from certain NCR guards.
I know you said this is your first playthrough... I'm not going to assume you're as slow as I was (it took me over 20 games to spot this), but just in case you're not sure what J_Fred is talking about when he mentions the high-level cave robbers: Most of the time when you get a random encounter on a map that has a cave mouth in the middle of a cliff face, you can actually enter the cave (though there's no visible exit grid) and it will be filled with random baddies. Most commonly it's typical wasteland fauna, but they can also contain bandits or even deathclaws. Around level 20, the bandits will pretty reliably have access to gauss guns when you run across them, though it's usually only 1 or two of them per cave.