GUMPS is just a quick mnemonic to use in addition to your landing checklist to help you remember some of the most essential items during descent to landing. Please, always use your printed checklists!
Gas (fullest tank)
Undercarriage (gear down and locked)
Mixture (full rich)
Propeller (high RPM)
Switches (fuel pumps, magneto check, landing lights, seatbelts, etc)
If you're flying a plane with a carbureted engines, you might run the C-GUMPS or Charlie GUMPS checklist. Charlie stands for Carburetor (heat on).
I'd like to reiterate that this is a basic acronym to use in addition to the standard descent, approach, and landing checklists. Mental checklists should not be used as a substitute for an actual checklist.
Have fun. Be safe. Happy Piloting.