Hi all,
I am supposed to design an LDO with 1.5V ( Fixed) output and a max current of 150mA. How should i find out what gain I need for my error amplifier. How can i decide upon the specifications for error amplifier like GBW, slewrate etc. can someone help me on this. any materials regarding the same would be very helpful.

Your gain is set by your line regulation(PSRR at 0Hz) or your load regulation, whichever is worse. Try the slow P or weak P corner at maximum temperature and this will tell you the worst case gain for a weak power device.
The bandwidth is set by either the max Iq available or your PSRR over frequency requirement(Audio usually requires 60 dB @20KHz) since you need to have loop gain at the frequency you want rejection.  There are all sort of tricks; dynamic biasing of certain key blocks in the LDO proportional to load current, ways to compensate the loop, interesting architectures for any load capacitor stability, etc... The smaller your load capacitor, the faster your linear loop needs to be in order to avoid the output voltage from dropping, dynamic biasing usually helps here a lot.