Do NOT replace it. I'm not a qualified electrician but I've studied up on it quite a bit (you would be surprised at how much cable can interact with house power). The reason there is a GFI in the garage is that is where people tend to over use the outlet (I've seen many people put both a freezer and a fridge on the same outlet). The GFI acts as a safety breaker before it gets to the primary breaker. This is now code. If the house is sold it would need to be replaced. The reason the breaker in the panel doesn't trip is the gfi is commonly a 15 amp limit before it trips while the breaker is a 20 amp. If it is tripping constantly you need to find what is causing the problem or move some items to a different circuit.