If your goal is to optimize the space positioning of some products on the shelf for an advertising topic you could adopt an eye-tracking method to extract the fixation points of the customer in front of the shelf, while he's watching the products. Metrics as NSS and AUC can be used to give a measure to the distance between the most important products and the real visual perception of the scene he looked.

Based on SERQUAL model, yes one can use gap scores at the indicator of customer satisfaction.

Thanks for the answers.