Conversion of the model from Brian1 to Brian2

Not quite, it should be

Cmm_weak_inh.delay = 'dm - dd + 2*dd*rand()'

or

Do I understand correctly that this is meant to copy the M→G connections so that G→M has the equivalent symmetric connections?
In that case, the most straightforward translation would be to exchange the source and target indices, by using:

Cgm_GABA.connect(i=Cmg_AMPA.j, j=Cmg_AMPA.i)

Another alternative would be to use a single Synapses instance with an on_pre statement to apply the spikes from M→G, and an on_post statement to apply G→M.