I've finally got everything set up and am ready to do my first Trainerroad workout.

I understand the concept(s) behind wanting to keep tire pressure and trainer tension constant from workout to workout.

What I don't understand (using Virtual Power) is how to know what gear to ride in. Since I am just using a speed/cadence sensor with Virtual Power, all of my results are going to come from how fast my tire is turning. Obviously, if I'm in my easiest gear, I'm going to look like a professional bike with a cadence of over 120 on all workouts. Consequently, if I'm on harder gears, my results are going to suffer as my rpms will be really low.

How do I reconcile this? Does the program tell you what gear to use?

Ride in whatever gear you're comfortable in. The power estimation is based on speed only not cadence. Your choice of gear has no impact on the workout.

+1

Some trainers (e.g. Kurt Kinetic) are more erratic in terms of tension on the tire compared to others (e.g. Tacx). I ride rollers (e-motion) and the power readout is always lower than actual power if I use virtual power. This is due to the power calibration on trainer road being based on a 180lb bike+rider and I am quite a bit over that.

^ I have seen the opposite. Power estimation by Trainerroad (I tried a few times) is higher than measured power (by Powertap), using a Cyclops fluid trainer.

I can believe that. That was my experience with the kurt kinetic unless I tightened the resistance unit screw so much that I thought the tire was going to burst.
The inside ride rollers on trainerroad are calibrated for a 700x23 tire and a bike+rider weight of 180lbs. I am way over that.

I have been using trainerroad for about 3 weeks. I just got the garmin vector pedals and there is a noticable difference at the same level of effort.

I will continue to use the heart rate monitor and speed cadence sensor since it is important that I stick with one platform for the remaining 3 weeks of the program.

I use a PM with TR, but I thought that the virtual power was based on speed and cadence... meaning, cadence is the only way it truly knows how to estimate the torque output... I could be wrong, but as the OP posted, he could be on a very easy gear spinning very fast, but if TR doesn't know that his spin, it can't calculate power.
At the end of the day OP, it is all relevant and will reamin consistent and you will compare your rides against your own #'s so you will improve

TR is based purely on speed, cadence doesn't matter. They have plotted resistance curves based on the power required to maintain a corresponding speed and that's what's used to convert to VirtualPower.