If the panel is mounted on bearings and well balanced, then you can easily get servo motors with the torque needed (I've seen r/c servos with 200+ oz/in torque - they aren't cheap, though). Since you mention the need to drive the gearmotor directly (ie, from an h-bridge), what you could do is remove the servo's control PCB, and solder wires directly to the motor in the servo (effectively turning it into a gearmotor). Otherwise, you are going to be looking at using a regular DC gearmotor. If this is a one-off project, then I would recommend you go with surplus gearmotors (my experience has been with Pittman gearmotors, which are very well built, have a wide range of torque and RPM outputs, with ball-bearing or oilite bushings on the output shafts - but they aren't anything like cheap brand new - however, surplus they can be found reasonable). Also, is your design fixed, or is it flexible? That is, do you need the motor where you have it (not a bad design insofar as keeping it out of the weather)? If you can change your design, you can open up other possibilities (such as the already mentioned linear actuator). You also haven't said anything about a budget, or if you are looking for new parts, etc; for instance, those Pittman motors I mentioned cost well over $100.00 brand new, but can be found surplus for $20-50.00 depending on the options (higher prices are generally those in really good condition with encoders on the output - which you may or may not need). I can also tell you where you could possibly get new gears or surplus gears - without knowing more about your budget, project flexibility, and whether this is a "science experiment" or something to see actual use 24/7/365 - its more difficult to make recommendations.