GPG Gearmotors are a good quality version of the standard motor. A standard motor is a series of motors which include Induction, Speed Control, Electromagnetic brake and Reversible motors. The are completely modular in design and can be fitted with a variety of different gearboxes ranging from 3:1 through to 200:1
The are a common replacement for the Oriental Motors which was the first factory to make them, there have since been hundreds of manufacturers copy them (way too many to mention here). There are two series of this motor, the early series 1 and the newer series 2. The series 1 had better heat sinking but the version2 looks more modern.