Without Gapps, your Android 5.1 device would function with basic AOSP (Android Open Source Project) apps but would lack access to the millions of apps on the Play Store or any Google-dependent features.
Would you like a device-specific walkthrough (e.g., for Nexus 5, Galaxy S4, or a low-end tablet)? Just tell me the model. gapps android 51
I recommend for 5.1 — just Play Store and core services. Without Gapps, your Android 5