OTAs, especially simple ones, lose gain big-time when
the output gain device, or load device, goes into its
linear (MOS) region; gm*Ro with Ro dropping when
output gets within ~Vgs(op) of either rail. I'd guess
that your useful output range on a simple OTA would
be ~ 0.5 - 1.3V for a 1.8V supply; outside this range
the DC gain (and Vio) will degrade markedly.
If you want to apply classical op amp characterization
to an OTA you need to include / specify a proper (i.e.
application-realistic) load to get voltage in, voltage
out basis. Now the application may have only some
other CMOS gate terminals as the load - but this is
for you to say, the OTA itself don't know nuthin'.
You might (if it's general purpose) check out a series
or matrix of RL (to what voltage?), CL to see across
what range of signal and load the amplifier is useful
or acceptable.