Not only is this needed, but it also needs good UX to make what's happening clear.

In the OVERWHELMING MAJORITY of situations, I want the CLOSEST gas station AHEAD (or on my current) route. It could be something as easy as a modal popup that says [BUTTON] in brackets:

[Shortest Distance] or [Nearest Ahead]

Where [Shortest] = "oh my .... I'm on fumes and need fuel NOW"
And
[Nearest Ahead] just does a quick check of your current route, a little balancing to find the shortest route with a 10-minute "lookahead" and presents the user with a list that INCLUDES:

[Along Route]
"Time (DIST) to (GAS STATION) adds (X DIST)"
Being able to SORT by the (X DIST), which would be the "miles added by deviation" then allows me to see the shortest add miles and how long".

When I'm on local roads, it doesn't matter as much. But, when I'm on a trip in unfamiliar areas, I usually want an upcoming fuel location that adds the least "exit .... back on" time or distance. I'm fine with driving 20 minutes if it's an addition of 2 miles, but want to avoid closer options that add 5 miles as long as it's not dire.