After 10 minutes, the battery life went down from 80% to 70%. That's 1% per minute. The battery life has been fluctuating a lot - some days I will end with 40%, others I will still have 80%.
My battery stats are no where near the numbers you're reporting with the use of GPS. On the contrary, my Apple Watch (Gen 1 Stainless) does really well with the use of GPS. Then again, it depends how long you are using the GPS for.