Both #m^3# and #"litres"# are units of volume. There are #1000*L# in a #"cubic metre"#, which is a very large volume.
#1*dm^3# #=# #(1xx10^-1*m)^3#, because #"d"# #-=# #"deci"#, i.e. #10^-1#.
And thus #1*dm^3# #=# #(1xx10^-1*m)^3# #=# #10^-3*m^3=1*L#.
#1*mL=1xx10^-3L#
And #1*cm^3=(1xx10^-2m)^3# (because #"c"-="centi, "10^-2)#, and thus,
#1*cm^3=(1xx10^-2m)^3=10^-6*m^3#
#=# #1/(1000)xx1/(1000)*m^3#
But #1/(1000)*m^3-=1*L#
And #1/(1000)*L-=1*mL#, as required.
Confused yet?
I don't know what you mean by #Gm#.