Unless things have changed since I lived in Japan, gas stations are usually not food options or combined with convenience stores like they are in the States. The main exceptions are highway rest stations on the national expressways. Most gas stations in Japan are full-service, where the attendants rush out and pump your gas, wipe your windows, and so on, so the customers don’t leave their cars.
Now convenience store food in Japan is very memorable, with a vast selection of prepared cold foods, warmable foods, bento box meals, and hot food options. You will see a LOT of things you would hard pressed to find in Japanese restaurants stateside unless you are in a city with a large Japanese presence. For example, you almost never see what the Japanese call “yoshoku” (i.e. “Western food”, but a Japanese approximation of it) in Japanese restaurants in the U.S, but it’s a staple of convenience store food.
I used to hit my local convenience stores (Lawson and 7-11) for: karaage (Japanese boneless fried chicken), and croquettes kept under the heat lamps, oden (they ladle the broth into your container, and you pick your ingredients (hard-boiled egg, fishcake, etc) by the piece. I also remember buying Nikujaga, Hayashi rice, and omuraisu.
Here’s an overview of convenience store food.
That being said, sushi is the one thing I wouldn’t buy in a Japanese convenience store is sushi, nosily because I had much better options to get better, fresher inexpensive sushi such as the fish/sushi counter at the supermarket, and chain conveyor-belt sushi restaurants.