Happens both with and without optifine. Unfortunately, I was unable to record a gif, but the title should be pretty self explanatory. The gateway simply disappears. It reappears if you step back.

Could you post an image? It's working fine for me...

Theres no glitch, it just vanishes. I'll try to record a gif if I encounter it again. Its probably connected to the age old bug in minecraft where tile entities stop rendering at the edge of your vision. Which, by the way, affects rift renders as well.

What do you mean it vanishes? If it's at the edge of your vision, then that's not a dimdoors bug, but rather a vanilla bug, there's no way to adjust the TESR render distance.

Heres a gif:

https://www.dropbox.com/s/8s1cv4ye8m8ejqa/glitch.gif?dl=0

Had to use dropbox due to file size.

Fixed in latest build (https://github.com/DimensionalDevelopment/DimDoors/releases/latest).