If I want to buy some gear motors that have high torque and high speed to move a heavy (about 10-20 pounds) robot at about 5MPH, what gear motor specs should I look for?
you need to understand how torque works (it's measured in a variety of ways - generally ounce-inch/inch-ounces, pound-inches/inch-pounds, pound-feet/foot-pounds, gram-centimeters, kg-centimeters, etc).
For example, if a motor is rated with a torque of 20 inch-pounds, what that means is that the motor could lift (up-down) a weight of 20 pounds with a lever attached to the shaft that was one inch long - measured from the center of the shaft. Conversely, it also means that if the lever was 20 inches long, it could only lift 1 pound. If the lever was 10 inches long, it could move 10 pounds. See how that works?