Gapps, the suite of Google apps that you have got installed on your Android phone is an essential thing for custom ROM users. Most custom ROMs are ships without Google apps package, so you need to download them separately and installed to get apps like Google Play Store. Open Gapps project is the best place for downloading Gapps for your custom ROMs.
Open Gapps Project is probably the easiest way to download the latest and correct Google Apps for custom ROMs. A number of different versions of the apps packages are available such as aroma, stock, full, mini, nexus, etc. You can choose a basic or full package to suit your exact needs. All packages should work fine on any custom Android 4.4.x or 5.1.x ROM.
From the developer “Open GApps is a Google Apps package completely developed by writing build scripts which allow for the automated creation of new up-to-date packages automatically. The development process is completely open-source and the goal is to have multiple contributors involved, to secure and reinforce the sustainability of Open GApps development.”
Currently only arm and arm64 platforms are fully supported. x86 platform still misses some apps packages in full and stock package. Also, x86_64 is under development. The latest download can be chosen by platform (arm, arm64, x86, x86_64), Android version (4.4, 50, 5.1) and package variant (aroma, stock, full, mini, micro, nexus, etc).