It does not mean that you can’t run a Custom ROM without these apps, no its not like that, however these are essential apps that almost every android users need, Play Store which have millions of free apps, Google Search, I am pretty sure everyone uses this one. If you’re one of those android users who are interested in rooting and flash Custom ROMs then probably you would need Gapps or Google Apps, as a Custom ROM like CyanogenMod, ParnoidAndroid, PAC-man, AOKP, SlimBean, OmniROM, Carbon and so on does not have these apps pre-installed. These apps includes Google Play Store, Gmail, Maps, Search, Chrome, YouTube, Drive, Keyboard and Google Hangouts.

Gapps or Google Apps are set of useful applications which comes pre-installed with almost every android device.