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you can control the gm(aspect ratio and bias current) of your input pair, ro of our loads (length, bias current) to get the required gain.
also depending on whether your input pairs are biased in saturation or subthreshold, gm could be lower or higher.
it depends on the accuracy you want. Gm is more affordable than rout of the loads. The better thing to do when setting accurate gains is to use architecture which provide gains given by the ratio of homogeneous quantities, such as resistances or transconductances.