It is possible to walk around on a gas giant if the character is in a city.
However, this does not even need to be the case for you. You can simply pilot your ship to the same position as the vehicle. Go to the entrance/exit of the ship, and you will be given the option to enter the vehicle directly (Aassuming that you are the pilot/commander or on the crewlist). This will allow you to enter your vehicle without actually being on the gas giant. You can then do the same to re-enter the ship once you've collected your items.