I’ve seen as low as 5c per gallon and as high as 38cpg with GasBuddy. It does link to your debit account; when you swipe, the full amount gets charged to GasBuddy, they process the discount, and the discounted amount is what gets charged to your debit account.
Upside requires you to charge your debit account directly, and then you get credited the discount in the app. Only taken Cashback once from Upside, as GasBuddy nearly always beats Upside on total discount. Upside seriously slices the discount if you try to pair with station/brand rewards.