I think that you may be barking up the wrong tree because it is impossible to buy a mountain bike for $300. Let me explain (this is kind of long).
There are 3 kinds of bikes normally called mountain bikes-
"Fat Tire Bikes" or FTBs are typically those which you'll find in discount stores or other retailers. They almost always look like mountain bikes with lots of bells and whistles, but they are intended for on road and on light trails only. This is usually disclosed by a sticker on the bike or in the owners manual. These usually sell for between $0 and $300.
"All Terrain bikes" or ATB's are a good choice for onroad, offroad, and more technical terrain (rocks and ruts). They are usually well built, as you go up in price they get better. If your budget is $300 don't buy a $200 bike as $300 is about the entry point for a decent bike. ATBs are priced between $200 and $700.
"Mountain Bikes" or MTBs are just that... put them in your car, take them to a mountain, ride them, and when you're done put them back in your car and drive them home. They should NOT be used on the road. These tend to be very solidly built and have features that make that type of riding safer and more enjoyable. Mountain bikes START at about $500 and go up from there.
You should avoid disc brakes on an entry level ATB or MTB unless you are prepared for a high maintenance interval. Instead, choose a bike in the same price range with "linear pull" or V brakes. There is little need for discs unless you race or play in the mud.
Also avoid full suspension (FS) unless you plan to spend at least $800. Keep in mind that suspension on a bike is for the same reason as on a car... not for comfort, but to keep the wheels on the ground for better handling. Every time you pedal a FS bike some of the energy you expend is eaten up in the suspension rather than going to the rear wheel. Instead, get a bike with front suspension and a rigid frame... you'll be miles ahead.
Also consider the complexity of disc brakes and FS. Exactly what kind of quality might you expect of these items on a $300 bike especially when it probably costs $300 just to replace 1 shock on a car?
If you will indulge me just a little more...
In the 1890s when the "ordinary" bike (bike with both wheels the same size) was introduced the average cost of a bike was $94.00 which translated to 6 MONTHS pay. Now, I don't suggest that anyone spend 6 months salary on a bike, but perhaps a week or 2 would be a better budget, don't you think?
Anyway, forget the GMC. If the Mongoose is a MongoosePRO you'll do OK but do NOT choose one that is branded Mongoose (without the "Pro").
Feel free to email me if you have any questions on other brands or sizing a bike, etc.
26 years in the industry