The smallest unit of NEO is 1, in other words: it is indivisible. The smallest unit of GAS is 0.00000001 (one hundred millionth). As long as you possess NEO (at least 1 NEO), GAS will be generated automatically. You can calculate how much GAS you would generate with x amount of NEO on https://neotogas.com/ or any other GAS-calculator.
For more information, please refer here