There's a sign visible at one point for about one frame when Gus is running, and it took me a solid four minutes but I managed to catch it for you all: https://i.imgur.com/8vKyR1j.png
I'm gonna guess the girl was joking. as a former gas station employee I can say you make a lot of weird jokes at customers in an attempt to be playful or have fun because the job fucking sucks. of course i could be wrong but the fact that she bought him the soda makes me think that. she realized the joke didnt land and was trying to make up for it
@dethia You have to remember this took place in Burnet, which is close enough to Austin to still be considered the sugar daddy/baby capital of the state/country.