Android 5.1 ROMs have come up which are based on CyanogenMod 12.1 or AOSP. Gapps consists of Google apps which is pre-installed on Android devices. However, Custom ROMs don’t come up with in-built Google Apps or Gapps, you are required to install Gapps separately on your device.
The Android 5.1 Gapps comes in various sizes by different developers. You can download Android 5.1 Gapps in a small package of 88 MB.