If you are custom rom user .. and wana use G Apps u need to flash a Gapps.zip for this..
For diff android version diff gapps available..
Opengapps most popular. You can download gapps from opengapps.org
|HOW TO FLASH GAPPS….
● Download current Gapps package as per yoyr requirements.
● And flash just after your rom zip.
|For Android 5.1 select this combination…arm-5.1-pico|
|For Android 6.0 select this combination…arm-6.0-pico|
|For Android 7.1 select this combination…arm-7.1-pico|