The slip activity and slip interaction in tensile deformation of peak-aged cast and extruded Mg-10Gd-3Y-0.5Zr (wt.%) at 250°C was investigated using in-situ scanning electron microscopy. Basal slip was the most likely system to be activated during the tensile deformation, while prismatic and pyramidal <c+a> slip also contributed to the deformation. No twinning was observed. The number of non-basal slip systems accounted for ~36% of the total active slip systems for the cast alloy, while non-basal slip accounted for 12–17% of the total deformation observations in the extruded alloy. Multiple-slip was observed within grains, and the basal/prismatic pairing type dominated the multiple-slip observations. Slip transfer occurred across grain boundaries and most of the slip transfer observations showed basal-basal type. The involved slip systems of slip transfer pairs were always associated with the same direction. The slip transfer occurred more easily at low angle boundaries (LABs) and boundaries with misorientations greater than 75°.