So, the gameplay... Well,
Garfield's Nightmare is a 2-D platformer whereby Garfield has overeaten before bed and has become trapped in a nightmare world. Four worlds, actually. He smashed his alarm clock in the real world and can't wake up, so the player must work their way through each of these nightmare worlds, defeating baddies and a boss and collecting a piece of the ruined alarm clock to eventually wake Garfield up.
The formula is very basic. You troll through really pretty worlds stomping on bad guys or avoiding them altogether, collecting pizza, donuts and coins along the way. By collecting 100 donuts, you are granted an extra life. Your pizza slices, of which you can carry up to three, indicate how much life you have. Run out of pizza and lose a life. Your Garfield's represent your "continues" and you get three of those. Collecting coins opens up the "sleepwalk door" which is a bonus level that can only be accessed when you have enough coins, which you find along the way or obtain by stomping bad guys.
You'll be doing lots of jumping, mostly. Sometimes, you'll have to backtrack to find a button to activate a platform. Other times may have you pushing or pulling a box to step up to an unreachable area. But mostly, you'll be walking and jumping, trying to reach a checkpoint, then finally the end of the level, of which there are generally five in each world.
There are also Mini-Games that are unlockable when you beat the boss of an area. They consist of Spider Phobia, collecting coins while avoiding spiders, Tap a Pet, a whack-a-mole type game and Midnight Morsel, where you catch falling donuts but avoid bombs. To be honest, I didn't get much enjoyment out of these mini-games and found them somewhat tedious.