This guide https://gamefaqs.gamespot.com/pc/199265-warhammer-40000-chaos-gate/faqs/15687 suggests that additional missions were once available on a site called The Portal (geocities). Does anyone know if these (or others) are available?

Have googled around without success..

Been playing for decades (wow, literally...), never heard of or came across such a thing. If you find something, please post back here!

No luck yet :)

Tried:
https://www.nexusmods.com/warhammer40kchaosgate?tab=popular+%28all+time%29
https://www.gamefront.com/search/files?q=warhammer+chaos+gate
http://www.moddb.com/groups/warhammer-40k-fans-group/downloads?sort=related-asc

I've also played with the WayBackMachine, but the search seems to be on URL not content..
... after stumbling across an archived page from oocities I found this:

http://geocities.com//timessquare/galaxy/6777/campaign.html looks like it may be what was referred to. An example mission is attached. It is only available on the 2009 snapshot.

I did find other info on making your own missions if anyone has the time and inclination:
https://www.youtube.com/watch?v=NqaIRWDLfmU
http://www.cheatbook.de/files/warh40kte.htm

I think there would be a lot of interest from the community