An effective strategy for offering services characterized by adequate levels of quality is to investigate on the single service aspects, to verify what needs to be improved or maintained, and what does not need particular resources. In this paper, we want to provide a practical instrument for investigating airport transport service quality, based on the opinion expressed by a sample of passengers. The case study is the International Airport of Lamezia Terme, a peripheral airport placed in the south of Italy; the data were derived from Customer Satisfaction Surveys conducted during 2015–2016. We propose a modification of the well-known Importance-Performance Analysis (IPA), by adopting as importance the weights determined through a Structural Equation Modeling (SEM) approach, and by plotting the gap between importance and performance on a visual graph, that we hold as easier to be read and more intuitive than the one used in the traditional IPA. The results obtained by the proposed Gap Importance-Performance Analysis (Gap-IPA) suggest that the attention of the airport operator should focus on the service aspects related to cleanliness. Besides, the methodology can show which aspects should be kept under control for avoiding to become critical issues of the service.