By the time of the Gathering, were all the remaining Immortals known to each other by sight? McLeod recognises Fasil in the MSG car park and knows Kastagir and the Kurgan, but had they all met previously through the centuries?
We can obviously confirm that MacLeod and the Kurgan had met prior to the Gathering, and the dialogue exchanged between MacLeod and Kastagir clearly indicates that they'd met before as well, likely on multiple occasions.
We can't be sure whether MacLeod and Fasil had met in person, but at the very least they'd clearly heard of one another, and may well have seen each other too. Either that, or they had a good idea of how many Immortals were left (i.e. very few), and could therefore guess who the other was through a process of elimination. It may also have been the case that Fasil had specifically tracked MacLeod down, like the Kurgan did in the Scottish Highlands, in which case he'd have known who to expect when he finally caught up to him, even if he'd never actually laid eyes on him before.
That said, we were given no indication of any prior meetings between Kastagir and the Kurgan, or Fasil and the Kurgan, and it doesn't seem terribly likely that Kastagir and Fasil would've survived, had they encountered the Kurgan previously (unless they were conveniently saved by outside inteference, like MacLeod was in 1536). That part is pure speculation, of course, but since we've nothing more than speculation to go on in that regard, my best guess would be that they likely hadn't all met before.