Background: Gestational diabetes mellitus (GDM) and periodontitis (PD) are chronic inflammatory conditions that may share metabolic and immune pathways. Evidence suggests an association between them, although results across studies remain inconsistent. This systematic review and meta-analysis evaluated the relationship between GDM and PD and examined whether GDM influences key periodontal parameters. Methods: A systematic search of PubMed/MEDLINE, Scopus, Web of Science, and Embase was conducted up to August 2025 following PRISMA guidelines. Observational studies comparing periodontal status in pregnant women with and without GDM were included. Periodontal status was assessed using probing pocket depth (PPD), clinical attachment loss (CAL), and bleeding on probing (BOP). Study quality was evaluated with the Newcastle–Ottawa Scale, and random-effects models were applied to estimate pooled associations. Results: Fifteen studies involving about 3800 pregnant women met the criteria. A significant association was found between GDM and PD (Odds Ratio [OR] 2.10; 95% Confidence Interval [CI] 1.64–2.69). No significant association emerged between GDM and gingivitis. Women with GDM showed increased BOP and higher PPD, indicating greater periodontal inflammation, while CAL did not significantly differ between groups. Conclusions: The findings support a significant association between GDM and periodontitis, suggesting that gestational hyperglycemia may enhance periodontal inflammation and early tissue changes. Incorporating periodontal screening into prenatal care may benefit maternal oral and metabolic health. Further longitudinal and interventional studies are needed to clarify causality and to explore whether periodontal therapy may help reduce risks linked to GDM.