I think there are a few things you can try:
If Taby seems docile (sounds like it is), consider taking it to a local vet or shelter to scan the cat and see if Taby is microchipped. If Taby has a microchip, you could then contact the owners and have them keep Taby indoors, or generally help keep her on their property (enticing with food, etc.).
As an altered version of completely locking Garfy in as Bella Swan suggested (which is still a viable option), consider keeping Garfy in only at night. Guard his goings ins and outs to ensure he's the only cat entering and exiting the cat flap. They also make cat flap doors that activate with a sensor you place on your cat's collar, so the flap will only let your cat in.
Here's my favorite option - Have you considered owning one more cat? It's apparent Garfy and Taby are friends, and having two kitties is always more fun. They can keep each other company, and it's not that much more expensive to keep two cats over one. Some vets will give you multi-pet discounts. Taby may also feel more comfortable in the house and let you sleep instead of panicking to get out. And consider re-naming Taby to Nermy ;)
