My question is this. I want to purchase a bike to help with my mountain bike training. I want to do a mix of road and gravel road riding. Could I use a gravel bike for both? Just change out the wheels or would I need to get one of each? Thanks
A gravel, all road, and/or cyclocross bike will all work fine. There are some subtle differences in geometry and handling between the three (as they are optimized for slightly different off-road use cases), but all three excel in mixed terrain riding and will work fine. Just make sure the frame has ample tire clearance.
In terms of tires again it all depends. I personally run large volume supple slick tires on the gravel. These work fine on the road and on gravel. Tread does little for traction on gravel as the knobs are not long enough to pierce through the layer of gravel to the solid ground beneath. Traction comes from the tire interacting with the top layer of gravel. Wide and low-pressure expands your contact patch to including more gravel and therefore more traction - that's what's important.
That said If I was mixing in more single track, or mud, I would probably move to a tread pattern and have a second wheelset with slick tires mounted for road rides. If the road riding included club rides I would probably have a narrower slick tire mounted on the second wheel.
Before buying any extra gear, I suggest just start riding. Establish your riding habits, before looking to more gear.
RULE 11: The proper number of bikes is n+1 where n is your current bike tally.
Cycling begets cycling.. with that in mind you should decide if you ever want to try your hand at road racing, cyclocross racing, or gravel grinding (racing). Each of the bikes are different with the road difference being more obvious than the difference between cyclocross and gravel bikes. So here it is... a true legit cyclocross bike will only have frame clearance for 700 x 35 tires (maybe even less as the UCI max tire width is 33), be stiffer than a gravel bike, and have an aggressive geometry designed for going fast for a limited time... speed over comfort. Gravel bikes tend to have slacker (more relaxed ) geometries and facilitate larger tires for more comfort over the long haul.
So Dale, I completely agree... you want to be your fastest on a mountain bike you need to train on the road (even gravel roads) and it is more fun to do that on a bike that is designed (not over designed) for that purpose. I personally have a cyclocross bike for that very reason. I can use it for gravel grinding albeit with a skinnier than desired tire, I can use it on the road with 700 x 28C tires, and I can race cyclocross all on the same bike with the same wheelset. What you need to decide is if you want a general purpose machine like a cyclocross bike or do you want to gravitate towards the road or gravel designs that will be more specific their domain but very likely work for other endeavors within some constraints.