installing outdoor GFCI from existing indoor circuit
Bathroom, kitchen and dining receptacles circuits are off limits. So is the laundry circuit. Circuits dedicated for appliances fixed in place are also out. Any other circuit with reserve capacity should be ok. The circuit must be grounded.
GFI protection is required outside as well as the weatherproof in use cover.
Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.