1. How much gas do you need?

(What does gase mean? Just gas or gasoline?)

2. I ate a piece of watermelon.
3. I ate two pieces of watermelon.
4. I ate a piece of watermelons.
5. I ate two pieces of watermelons.
(Which ones are correct?)

2 answers

The word "gas" is usually short for "gasoline" -- especially when referring to cars. The word "gas" can also be referring to natural gas, a different fuel that is used mostly for cooking and heating.

2 and 3 are correct; 4 and 5 are incorrect.
They all are correct gasoline is a short word for gas so all the of the answers are correct