In order to measure the ac gain you have to inject a frequency-dependent disturbance: insert a (unit) ac voltage source either in series with vref or between the inductor L0 and the Vplus input.
BTW: the upper node of L0 in any case needs grounding via a (large) cap.
PS: Usually not the gain of an LDO is interesting, but e.g. its Power Supply Rejection Ratio PSRR. In this case, the unit ac voltage source is inserted in series with the power supply (Vdd_High). PSRR = dB20(VF("/vout")/VF("/v unit ac voltage")).
As VF("/v unit ac voltage") ≡ 1 , PSRR = dB20(VF("/vout")) < 0 , e.g. -80 .. -20dB .
In order to measure the ac gain you have to inject a frequency-dependent disturbance: insert a (unit) ac voltage source either in series with vref or between the inductor L0 and the Vplus input.