the general 5 star gas cap can easily be popped open without a key.
grab a small screw driver or a pick, push the cap down towards the tank, and use the screwdriver/pick to push back the latch that holds the cap closed.
you have to get low and look between the cap and the tank to see the latch, but once you know how to do it it's cake.
push towards the front of the bike to release the latch. (edited)