Managed to figure out the solution, although it doesn't seem very logical!
a #Doesn't work
# Object of class GDf
a[1:nrow(a),1:ncol(a)] # Works
# a b
# 1 1 4
# 2 2 5
# 3 3 6
# NA NA
str(a[1:nrow(a),1:ncol(a)]) #Shows that this is a data frame
# 'data.frame': 4 obs. of 2 variables:
# $ a: num 1 2 3 NA
# $ b: num 4 5 6 NA