Do you mean Vo/Vi < 1, or Vo/V1 < 1 ?
From what I see, the left half of the circuit is a class-AB output stage. The right half of the circuit is unity-gains stage. Combining as a source-follower output stage, the output voltage Vo tracks V1. When the loading RL is finite, there will be current flowing through RL. This current is supported by M1/M11, or M2/M22. More current through transistors means a bigger Vdsat. Therefore, when V1 increases, Vo doesn't increases as much as V1 due to the bigger Vdsat. So the gain is a little bit < 1 .... actually it should be very close to 1.
On the other hand, Vo/Vi does have gain... gain can be > 1.