I agree with the others in that a .22 is the best way to start out. You can get a brick of ammo (500 rounds) for somewhere around $10, and shoot all day. And if you get a good .22 (Ruger, Browning, Smith & Wesson etc.) you will use it forever. You never "outgrow" a .22. I've been plinking with a .22 for the last 40+ years and you never get tired of them. Also great for small pests, cans, and golf balls.
When you are ready to get a centerfire gun, the GP100 you were looking at is a great gun. Built like a tank, and your grandchildren will thank you for it someday. It will need a trigger job to be really smooth, but well worth the money. A .357 is a great choice for starting out (and finishing for that matter) because you have 3 power loads to choose from. You can start out with .38 Specials, move on up to .38+P for a good defensive load, and then there is always the .357 mag. for the ultimate defensive cartridge (125-gr. JHP), and with 158 to 180-gr. loads, you can hunt deer with it. (The .44 mag is better for deer, but a gazillion deer have been taken with .357s and it will do the job.) The .38s are also good for small game (squirrel, rabbits, *****, et al.). Any of the medium/large frame Smith & Wesson revolvers (my favorites) will works just fine, too, but you will have to pay a wee bit more unless you find a good used one.
.44 mags are the most fun you can have behind a trigger (especially single actions), but this is NOT where you want to start. Just like you don't learn how to drive at a NASCAR race.
Back to .22s for a moment. Both Ruger and S&W make fine .22 revolvers as well as automatics. Automatics can be fickle about the kind of ammo they like. You can have two guns of the same make and model. One will like a certain type (brand) of ammo, and the other will jam all the time with it. Some folks have Ruger .22 autos that will shoot anything. Mine would only shoot CCI Blazers without jamming. My S&W 2206 semi-auto will shoot anything but Federal ammo. You're just gonna have to find which one they like. A revolver, however, will shoot anything you stick in there, any brand, shorts, longs, or long rifles. Even .22 shotshells. Sure, they are a tad slower to shoot, but what's your hurry? You're plinking to have fun!
OK, I'm sorry. I've rambled on enough. Let us know what you decide on. It's kinda hard to go wrong if you're havin' fun!