Background/Objectives: The prevalence of gestational diabetes mellitus (GDM) is increasing at an alarming rate worldwide. Delayed management can lead to adverse composite outcomes for both mother and her offspring. To our knowledge, the clinical association between glycemic parameters and the results of the non-pharmacological GDM approach remains limited; thus, this study aimed to address this important clinical issue in the literature. Methods: This was a retrospective cross-sectional study of 174 Vietnamese pregnant women with the positive oral glucose tolerance test (OGTT) for a high fasting glycemic parameter at Hung Vuong Hospital from 04/2022 to 07/2022. This study aimed to evaluate the success rate of GDM with an elevated index of fasting glycemic concentration which was managed after 2 weeks of a dietary regimen combined with adequate physical activities and to reveal its related factors. Results: Out of 174 singleton pregnancies that met the inclusion criteria, 103 GDM pregnant women were successfully managed after 2 weeks of monitoring (59.2%; 95% confidence intervals (CI): 51.9–66.5). The study revealed a fair correlation between the corresponding test of blood glucose at OGTT and after 2 weeks of GDM management (r = 0.270–0.290, p < 0.0001). The GDM pregnant women with an elevated fasting glycemic parameter and with any of elevated 1 h or 2 h blood glucose levels and in cases of three elevated glycemic parameters (fasting, 1 h, and 2 h blood glucose at the initial results of OGTT) reduced the success rate of glycemic control to 56.5%, 49.2%, respectively, compared to the group with solely a high fasting index of blood glucose (69.6%). The pregnant women who participated in high-intensity sports activities related to a two-fold increase in success rate compared with the group engaging in light and moderate-intensity physical activity. Conclusions: The success rate of glycemic control in GDM women was highest in the group with solitary fasting hyperglycemia and lower in the contributory groups with two and three high parameters. The application of diet therapy plus physical activities among GDM pregnant women is potentially necessary to improve the effectiveness of treatment, minimize adverse pregnancy outcomes, and reduce substantially the hospitalization rate.