The bill or cost charged for gas used in a home or building (city gas or LPG); part of utility expenses.
冬は暖房をよく使うのでガス代が高くなった。
Because I used the heater a lot in winter, the gas bill went up.
今月のガス代は請求書で確認しておいてください。
Please check this month's gas bill on the invoice.
光熱費を節約するために、ガス代を下げる工夫をした。
I took measures to lower the gas bill to save on utilities.
Colloquially used to mean the cost of gasoline (petrol) for a vehicle; gas/petrol expenses.
遠出するときはガソリン（＝ガス）代を多めに見積もっておいてね。
When going on a trip, estimate the gas (i.e., gasoline) cost a bit higher.
ガス代が高くついたから、今回は電車で行こう。
Since the petrol cost would be high, let's take the train this time.
同僚にガソリン代を立て替えてもらった。
A colleague covered my gasoline cost for me.