How much does gas cost to power boat from Hawaii to California?

First- very few vessels able to go from Hawaii to California run on gasoline. Most run on diesel fuel. So I am going to assume you really meant diesel.

An efficient 48' "trawler-yacht" capable of making the passage direct gets about 3 n. mile / gallon running at 7 n mile/hr. It is around 2300 n mile from Honolulu to San Diego.

This means that the trawler would use about 770 gallons of diesel for the passage alone.

At $3 / gallon for diesel that works out to about $2300 for the passage alone.

This calculation ignores the westward 1/2 n mile/hr North Equatorial Current and the headwinds expected in passage from Hawaii to California. This could radically increase the fuel consumption. Going from California to Honolulu rather then from Honolulu to California would use these to decrease fuel consumption.

Performance numbers published by Kadey-Krogen for their "NorthSea" 48 Trawler. Your mileage may vary.