Despite being basically a full cube, blocks behind end gateways are not culled as though one were:

This includes other end gateways:

I believe the reason for this is because the end gateway uses a block entity renderer. So if you go too far, it will stop rendering.
However unlike most block entities, it stops rendering after you are 256 blocks away, instead of 64.
Honestly couldn't tell the difference while testing, so I might just cull it and see

Now in the latest release: https://github.com/FxMorin/MoreCulling/releases/tag/v0.21.0-1.20.x