any suggestions on what motor should be used to lower the bridge/ramp? 
A servo (just kidding) 
I would use a window motor.
Any motor will do. The question is how long of a lever arm are you using (IE how much torque do you need) and how fast do you want it to go (how much you need to gear it down to attain that torque).
I second that, a window motor won’t back-drive so you can lower the bridge and not worry about having to put power to your motor in order to keep the bridge down.
I’d be a little concerned with a window motor, as (depending on the mechanism being used) the bridge manipulator may need to change angle as you approach the bridge (once its tipped)
How about pneumatics?
The correct way to answer this question is by doing the math. Figure out how much torque you need to lower the bridge, and then plan accordingly.
If this confuses you, talk to your physics teacher or an engineering mentor on your team. The problem is high-school level physics to determine the torque to lower the bridge, the force necessary to create that torque with different lever arms from your robot, and also to determine the various times that (ideal) motors would take to perform the task while creating that force.