Can I submit a list of differentially expressed genes, as log2 fold change values, coming from two different conditions to have a quantitative representation of pathways involvement?

If you want to use your log fold change data directly, you may use the Reactome Cytoscape app, described here (search for “Perform GSEA analysis”). This feature will sort your genes based on log fold change and then calculate a score for each pathway using GSEA’s ranked gene list input.