It depends on what style and level of language you are aiming for. Using 'gas' to refer to fuel implies that you are writing colloquial spoken American English, in which case 'stop some place to buy some' would be perfectly valid. You could also write it as 'someplace'. If you are aiming for textbook correct English, follow the advice of the previous answers and use 'somewhere' instead.

While that's perfectly understandable, you might want to say "We'll have to stop SOMEWHERE" instead of "at some place". Besides, there's no need to repeat the word "gas", so I'd just go for: "We're running out of gas. We'll have to stop somewhere to get some."

Yes!

Yes!

Another option would be, "We're running out of gas. We'll have to stop somewhere to fill up."