(1) Background: The introduction of highly effective CFTR-modulating therapies (HEMT) has changed the course of the disease for many people with Cystic Fibrosis (pwCF). Attention previously focused on life-threatening conditions of the respiratory system has broadened, bringing the involvement of the digestive system into the clinical and scientific focus. This emphasized the need for sensitive tools to capture and quantify changes in abdominal symptoms (AS), ideally applying patient-reported outcome measures (PROMs). (2) Methods: The present review focuses on studies addressing AS assessment deriving from the multi-organic abdominal involvement in pwCF. Among 5224 publications retrieved until Nov. 2022, 88 were eligible, and 39 were finally included. (3) Results: The review reveals that for a long time, especially before HEMT availability, AS in pwCF were assessed by single questions on abdominal complaints or non-validated questionnaires. PROMs focusing on quality of life (QOL) including a few GI-related questions were applied. Likewise, PROMs developed and partially validated for other non-CF GI pathologies, such as chronic inflammatory bowel diseases, irritable bowel syndrome, gastroesophageal reflux, constipation, or pancreatitis, were implemented. (4) Conclusions: Only lately, CF-specific GI-PROMs have been developed and validated following FDA guidelines, showing high sensitivity to changes and capturing marked and statistically significant reductions in the burden of AS achieved with HEMT implementation.