If you are missing Play Store, GMail, and other Google Apps after installing a custom ICS/Jelly Bean ROM such as CM9, CM10, and AOKP, it’s because these ROMs require you to install Google Apps separately.
When CyanogenMod ROM first came out with Nexus One, Google asked CyanogenMod team to not include Google Apps in their ROMs and sent a “cease and desist” letter.
Because of this, since September 2009, many CM9/CM10/AOKP custom ROMs have the Google apps installed separately.
If you are missing Google Apps for your custom ROM, you can install these in ClockworkMod Recovery (or your favorite recovery like TWRP):
Above GoogleApps are compatible with 99% of Android devices/custom ROMs.