10 or 12 gauge wire? (electrical)?

I am finishing an outdoor electrical project that has been abandoned by our electrician. The circuit consists of a 20amp GFCI breaker, 2 switches, and 5 outlet (one is GFCI). The total length covered is about 300 feet. The circuit will power 3 low powered lighting transformers, a small fountain pump, and any... show more I am finishing an outdoor electrical project that has been abandoned by our electrician. The circuit consists of a 20amp GFCI breaker, 2 switches, and 5 outlet (one is GFCI). The total length covered is about 300 feet. The circuit will power 3 low powered lighting transformers, a small fountain pump, and any accessories that may be plugged in (lamp, hedge trimmer, radio, etc.). Anyhow, he left both 10 and 12 gauge copper wire for the job; I don't know if he was planning on using both (like 10gauge supply then 12 from the switches?), but I am wondering which gauge is appropriate for the job? I assumed it would be 12 gauge (as with normal household wiring) but just wanted to make sure.

Thanks!
Update: Little confused about the 300 foot run. It's 300 feet total length.. meaning... Breaker to first outlet (supply line)(25-30 feet). Outlet to switches (5 feet). Circuit 1 to outlet 1 (40 feet), Outlet 1 to outlet 2 (40 feet)... Circuit 2 from switches to outlet (150 feet).. I guestimated the 300.
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