I have done gas station shops for several different companies, different brands, and different types of shops. NONE have been the same. Even shops for the same brand but with a different company are different. Sooo, really, nothing we can tell you will apply to EVERY gas shop whether its a mystery shop, mystery with reveal, or audit.
Generally speaking, though, a mystery shop usually involves an evaluation of the public restroom (if available), cleanliness and stock levels of the convenience store (if available), friendliness and appearance (description of employee and clothing) of the employee (if an attended station) and possibly the EXACT greeting and closing given by the employee, and an evaluation of the exterior amenities/services (again, if available) which typically include whether towels, squeegees, and fluid are available, content of pump toppers, if pay at the pump is available, condition of curbs and bollards, are prices lit on the ID sign, etc. Some locations require a receipt, some do not. Some locations require a gas purchase, some do not. Some locations reimburse for purchases (up to a set amount), some do not. And pictures of the restroom, pump, price sign (including hi-rise and off property signage for the location), building, canopy, curb appeal, and anything amiss or not compliant, or any combination of the above. Pictures of the interior of the convenience store are not required, except for the restroom. All done without calling attention to yourself or letting the gas station attendant see you taking the pic, of course.
A mystery shop with reveal typically involves all of the above evaluations, but not necessarily the exterior pics, before going back inside to present the LOA (letter of authorization) and getting permission to take additional photos. These photos MAY include pictures of the interior stock levels, dirty areas, and/or employees and more detailed pics of the exterior. Since the employee now knows you're taking pics, you don't have to be "sneaky" about getting the pics, but you do have to be careful of traffic on the lot.
I've only done an audit twice, so I'm not going to be able to give much advice on this. On the ones I did, they were more like mystery shops with reveal in that I did the mystery shop portion first, then revealed before taking more pics. I was looking to see if the location had the correct color curbs, bollards, canopy poles, ID sign poles, what POP was in the pump toppers, how the ID signage was configured, if the canopy graphics were in the proper place and the right color, etc. I think some audits involve checking stock levels inside the convenience store and possibly checking temperatures on the heated food, etc. I've not done any of those shops.