The research aimed at testing the combination of GAM/M-GAM models (Geosite Assessment Model/Modified Geosite Assessment Model) on selected geoheritage sites (geosites) of great scientific significance and geotourism potential. Testing was done on eight sites in the City of Belgrade (Serbian capital), an area which has significant potential for geotourism development in typical urban conditions. For this purpose, an assessment scale was used to highlight differences and similarities between main and additional values of the observed geosites. The modification of the original GAM model is based on the inclusion of visitors' opinions regarding the importance of indicators in the assessment process. The assessment was done by using both GAM/M-GAM and its results were analyzed and compared afterwards. The analysis has successfully identified locations and features of geosites that require action for maintaining or increasing their overall value and function. Moreover, the principal aim of the paper was to analyze the relevance of each sub-indicator for the assessment process by introducing the importance factor in the modified model. The authors were able to point out those values of principal importance for geosite visitors, as well as to attach a different relevance to sub-indicators, which can influence the position of the geosites in the GAM/M-GAM matrices.