Given that the Gordian Knot, which was cut by Alexander the Great in terrestrial history, appeared in Planescape as an artifact "cut by a powerful leader on a prime world", does this suggest that our own dear old Earth appears in the Planescape setting as a prime world? And if so, are there any campaigns set there, and what historical period is it in when all the stuff in D&D campaigns are happening on Toril?

The name of artifact doesn't mean it's from Earth. There are many such items in games you can simplify like that, but it's just an easter egg for those who know legends.

You could equally ask if blind Tiresias came on Ulysses's boat in Sigil.

ironbeard, it also says that the know dissapears and reappears on different worlds where it is needed, maybe alexander needed it or sumthin, doesnt necesarily mean that earth is a prime plane. but then again it could be, but if it was wouldnt proper everyday magic exist? i mean even with the persecution of all "witches" and wiccan religions there would still be some people who could cast magic.

all of the planes have curses of some sort. ours was obviously no magic and the 80's.

Lol! I suppose I read too much into their technique of borrowing a name and story from Earth history/mythology to make it recognisable. I think it would be cool if you could have adventures on analogues of our world, though. Of course you'd have to dump mages in a fairly civilised area/epoch, so they can survive by their minds without magic. "You see five men with guns, they are disturbed and command you to put your hands above your head"
"I fireball them"
"It doesn't work"
"I take my wand ...."
" You are seen reaching into your pack, they shoot you."