Yes, you get 5% back on gas. BUT! Your rebate is limited to 2% of your total monthly retail purchases. So, if you use the card just for gas, you'll be limited to a 2% rebate.
So, spending $100 on gas in a month and $0 on other purchases will get you a $2 rebate.
However, if you spend $100 on gas in a month and $1000 in other purchases, you'll get a $5 rebate (5%) for the gas and a $10 rebate (1%) for the $1000. That's $15/$1100 which is 1.36% total cash back and therefore below the 2% maximum.
If you were able to do the math and calculate how much you needed to spend on gas and other purchases, you could get basically 2% back on everything to buy on the card.
Long story short, Discover More and AmEx TE blow this card out of the water.