Ice sheets and their subglacial environments may represent an important source of methane (CH₄) and carbon dioxide (CO₂), potentially contributing to the atmospheric burden of these greenhouse gases. In this study, we investigate CH₄and CO₂ production, transport, and consumption at and near the sediment-ice interface beneath the Greenland Ice Sheet, utilizing basal materials from two deep-drilling projects. At the Camp Century site, on the northwestern margin of the Greenland Ice Sheet, vertical gas profiles (N₂, O₂, Ar, CO₂, and CH₄), combined with microbial DNA analyses, indicate that CH₄ accumulates in subglacial environments either from the release of gases from buried soils and sediments beneath the ice sheet or via <em>in situ</em> methanogenesis. The CH₄ then diffuses across the sediment-ice interface and undergoes partial oxidation to CO₂ within debris-rich ice layers, contributing to the pronounced CO₂ accumulation near the interface. CH₄ and CO₂ remain at higher concentrations than atmospheric levels for several tens of meters in the upper section of the basal ice sequence, suggesting mechanical mixing possibly during advection from inland. At the GRIP site, located at the summit of the Greenland Ice Sheet, biologically derived CH₄and CO₂ in basal materials is transported into the ice predominantly through mechanical mixing processes such as shearing and folding, although diffusion across the sediment-ice interface could also occur in the unsampled bottom section. There is no evidence of CH₄ consumption by methanotrophs at GRIP, suggesting that variations of bed conditions, ice dynamics and the nature of the organic material control the fate of CH₄ produced in the subglacial environments.