When you use your Earnin Card at a self-service pump, the gas station will put a hold of $100 on your card account. This occurs because gas stations are unsure how much gas will be purchased when you first swipe your card. The $100 hold ensures that there are enough funds on your card to pay for gas.
When you're finished pumping, the station will charge you for the actual purchase amount, and the $100 gas hold will be released once the transaction is completed, which can take several days.