GApps is a package of Android programs that you need to flash and install every time you install a new custom ROM because otherwise you won't be able to use them on your phone.
This package will install all the Google-based applications on your device so that you will be able to use Gmail, Gtalk, Google Voice, Calendar, Google Play Store, Google Maps, Goggles, Google Contacts and all the other applications you need to correctly use your phone. These apps can't be integrated in custom ROM packages because it breaks the licensing restrictions and you cannot integrate them with CyanogenMod installation.
As you know, then CyanogenMod doesn't need the GApps package to work but, you need it, after installing CM, in order to be able to install apps from Google Play Store, to use Google Maps, Calendar, Google Plus, Google Talk and all the other tools offered by Google that are required on a daily basis for a better Android experience.
Download Google Apps for any Android ROM:
Android 6.x.x Marshmallow and CM 13-based ROMs:
For Android 5.1.x Lollipop and later versions:
For Android 5.0.x Lollipop:
- Download Gapps for CM 12 Android 5.x Lollipop ROM here.
- Download Gapps for Paranoid Android 5.x Lollipop ROM here.
- Download Slim Gapps for SlimLP ROMs here.
For Android 4.x and older versions:
- Download Gapps for Paranoid Android 4.x ROMs from HERE.
- Download Gapps for all Android 4.4 KitKat custom ROMs from HERE.
- Download Gapps for all Android 4.3 JB and CM 10.2 ROMs from HERE.
- Download Gapps for all Android 4.2.2 Jelly Bean based ROMs from HERE.
- Download Gapps for Jelly Bean Android 4.2.x / CM 10.1 from HERE.
- Download Gapps for Jelly Bean Android 4.1.x from HERE.
- Download GApps zip file for other Android versions from HERE.
- this file has to be flashed after you've successfully installed the custom ROM firmware on your Android device.
If you wish, you can install only the Google Play Store and after that you can choose to download the rest of the Google apps from the market.