Which version of Gapps to install for Lineage?
You can download Gapps package for your lineage from this website source opengapps.org.
Site contains several fields such as :
1.Platforms:

ARM64 - For 64-bit devices
ARM64 means the devices that are running 64 bit operating system/processor.

ARM - For 32-bit Devices


ARM means the device that are running 32 bit operating system/processor.

x86
for Intel Smartphones like Zenphone .

x86_64
This is a very uncommon, these are used in some android emulators.
You can check this hereAntutu


2.Android versions:

9.0 Pie

8.1, 8.0 Oreo

7.1, 7.0 Nougat

6.0 Marshmallow

5.1, 5.0 Lollipop (Opengapps supported up to Lollipop in 2019)

4.4 KitKat.(Opengapps supported up to KitKat in 2018)


3.Variant:

Aroma: Graphical installer of the super package while in recovery menu, it allows user to select which applications to install.

Super: Includes all Google Apps that were ever shipped on a Google device.

Stock: Recommended package that contains all Google Apps that come standard on latest Nexus/Pixel phone.

Full : Very similar to Stock version.

Mini : Smaller set of popular Google Apps with extra functionality.

Micro: Limited set of Google Apps with Gmail, Calendar Google Now with extra functionality.

Nano : Minimal installation with extra functionality.

Pico : Minimum with Google Play functionality.
(Each package varies with file sizes and included Google Apps)


Your /system folder capacity decides which version is suited for you.
You can check your capacity using this app Disk Info. Then select your Gapps variant, and install it. And, I would recommend Aroma so that you can decide which apps to be installed during installation at recovery.(N.B:Aroma had issues with some recovery versions, but, it worked on TWRP 3.0.2 on my redmi 1s).
Note: Please mention which is your device. I have tried pico version in my Redmi 1S, because of lesser system capacity(Pico version includes apps like Google Play and Google Play Services). There after, I installed Google app as user app(from play store) and can run 'OK Google' voice command.