That golf cart battery will have a couple hundred watt-hours. Divide that by the watts you'll use and you get hours of use. Real world performance will be less. How long do you need to run off the battery for? At 6V, you'll need to convert to whatever your laptop is running on and the Starlink ...

I have two GC2's and 250W of PV in my travel trailer. It works fine. The batteries are really heavy though.