In Days Gone, you will eventually run out of gas; it's inevitable. So when it happens, you will need to search for more gas to fuel your motorcycle. Luckily you can find gas in a ton of places in the game, and nearly every unique location will have a gas canister you can use. You can also find gas dispensers and tanks that can be used to refuel your motorcycle.
Both will usually be at marked Gas station locations. However, these locations will also hold freakers or bandits lurking around.
Because of that, you will need to get rid of them first before you can use the dispensers to refuel your bike. Once the hostiles are dealt with, you will be free to use the dispensers or tanks to refuel your bike, and to do this, simply drive up to them.
Along with these locations, you can also use a gas tank at the first safehouse at O'Leary Mountain to refuel your bike for the time being.
But if you are desperate and have enough camp creds, you can also use a mechanic to refuel for a price of 200 to 100. You can find these mechanics in any of the larger settlements, so making a trip to them might be worthwhile in the long run.