Obesity is a multifactorial disease linked to chronic inflammation, metabolic disorders, and gut microbiota dysbiosis. Bariatric surgery (BS) and endoscopic sleeve gastroplasty (ESG) are effective for sustained weight loss and comorbidity improvement but may cause gastrointestinal and nutritional complications. This narrative review, informed by a structured literature search, synthesizes evidence on gastrointestinal side effects, gut microbiota alterations, and nutritional management after BS and ESG. Literature searches in PubMed and Scopus, without time limits, included English full-text articles on postoperative symptoms, microbiota changes, and nutritional outcomes. Bariatric procedures (e.g., Roux-en-Y gastric bypass, sleeve gastrectomy) and ESG are associated with adverse events such as dumping syndrome, GERD, nausea, and micronutrient deficiencies. Surgery induces profound shifts in gut microbiota composition and diversity, contributing to improved metabolic regulation. ESG, though less invasive, produces comparable microbial changes with a favorable safety profile. Nutritional management—progressive diet protocols and supplementation—is critical for preventing deficiencies and sustaining outcomes. Mediterranean-style diets appear more sustainable than high-protein regimens. Study heterogeneity, small cohorts, and limited long-term ESG follow-up reduce generalizability. Multidisciplinary care integrating surgical or endoscopic approaches with personalized nutrition and microbiota modulation is essential to optimize outcomes in obesity management.