Many install custom ROMs on their Android phones to escape Google, but there are also many using custom ROMs on, for example, Huawei phones that don’t come with Google pre-installed. But as Android users you have the option to, or not.
There are also more variants than just the GApps suite itself, such as Open GApps, NikGApps, FlameGApps, and more. The linked article is a good guide to the various options you have, and why you may want to consider them.
The article also leads into how to go about doing this with the TWRP application.
See https://www.xda-developers.com/download-google-apps-gapps/