The gluten free diet (GFD) has a high glycemic index and low-fiber content, which potentially influences glycemic excursions in type 1 diabetes (T1D) and celiac disease (CD). Participants in this case-control study of youth with T1D+CD (n = 10) and T1D only (n = 7) wore blinded continuous glucose monitoring systems for six days. Blood glucose levels (BGLs) were compared between groups for each meal, including pre-meal, peak, 2-hour postprandial and time-to-peak. Participants consumed a test-breakfast of GF cereal and milk for three days and kept weighed food diaries; nutrient intake was analyzed and compared to national recommendations. Youth with T1D+CD had shorter time-to-peak BGL (77 vs 89 mins, P = 0.03), higher peak (9.3 vs 7.3 mmol/L, P = 0.001) and higher postprandial BGLs than T1D (8.4 vs 7.0 mmol/L, P = 0.01), despite similar pre-meal BGLs (9.2 vs 8.6 mmol/L, P = 0.28). Regarding test breakfast, greater pre and post-meal BGL difference correlated with longer CD duration (R = 0.53, P = 0.01). Total energy and macronutrient intake didn't differ between groups; however the majority of participants collectively had inadequate intake of calcium (76%), folate (71%) and fiber (53%), with excessive saturated fat (12%) and sodium (>2,000 mg/day). The GFD is associated with greater glycemic excursions and inadequate nutritional intake in youth with T1D+CD. Clinical management should address both glycemic variability and dietary quality.