K, well basically, you want to use H-C(=O)-Cl, but unfortunately, this is not stable, which is where the Gatterman-Koch comes in. It just creates the species in solution.
But anyway, let's go about it another way. The strategy is to create the aldehyde and then add the bromide because the carbonyl is meta-directing. Then once we have the bromide in place, we can reduce the carbonyl to the methyl group. Somehow we have to make that carbonyl species. So let's do it this way:
Benzene + FeBr3, Br2 ------> bromobenzene
bromobenzene + Mg in ether -------> phenylmagnesium bromide
phenylmagnesium bromide + CO2 -------> benzoic acid (Grignard)
benzoic acid + LiAlH4 --------> benzyl alcohol (Reduction)
benzyl alcohol + PCC (pyridinium chlorochromate) ------> benzaldehyde (Oxidation)
benzaldehyde + FeBr3, Br2 -----> m-bromobenzaldehyde
m-bromobenzaldehyde + Zn(Hg), HCl ---------> m-bromotoluene (Clemmensen)