GApps aka Google Apps is the collection of Google Apps like the Play Store, Google Plus, GMail, Google Maps, YouTube, Google Search, Google Music, Google Hangout, Google Calendar, Google Drive and Google Now. Normally these apps come pre-installed on any Android Phone.
On the other side, when it comes to Android, it’s all about playing with root and custom ROM. If you are an Android geek, then you might know that Android Custom ROM doesn’t include any of the above mentioned Google Apps and you will have to download it separately. This is where the GApps comes into picture and much essential for those who flash their phone with Custom firmwares. To make things simple for you, we have gone through various GApps version and depending upon what ROM you flash your phone, download the GApps accordingly.
There is GApps for Lollipop, GApp for Kitkat and Google Apps for older Android version available.
GApps for Android 4.2.1
GApps for Android 4.1.x
GApps for Android 4.0.4
GApps for Android 2.3.7
How to Use
All of the above link includes GApps in ZIP format. You shouldn't modify them instead copy them to your phone external storage. Furthermore, the ROM flashing instruction will also mention how to use GApps. Once you flash your phone with any custom ROM, you will have to flash it with GApps, so that the above mentioned Google Apps gets installed on your device.
Note: Also don't modify any of the above ZIP files.