I'll try to clear up what they had to the best of my memory (And knowledge of grey knight/tau stuff)
a Hammerhead with an Ion Cannon, but more likely it was the other big battlesuit mentioned
Yeah the one with the large blast was indeed a big battlesuit, unsure of what kind it actually was but it was either AP3 or AP2 Large Blast because it removed the armor saves of my marines which is 3.
I am surprised that your tourny allowed someone to field a forgeworld unit with experimental rules and not have to explain it to you first.
So it was this stores second time hosting a warhammer 40k tourney so they are still working out what they can/will/want to allow and what they'll restrict. For instance the Skitarii got some restrcitons from memory they had to do with the amount of warlord like traits they got or something along those lines (sorry if that makes no sense i don't remember the skitarii related stuff, never fought them so it never came up again). As for the forgeworld unit with its special rules, well i wasn't aware it WAS forgeworld until i asked about the walker because it looked great, He never endeavoured to explain its rules/abilities and i never asked, didn't know they had a shit ton of custom rules thought they ran based of existing 40k rules with only slight modifications. The guy running it was aware it was forgeworld but allowed it (bare in mind this isn't a dedicated 40k store so that's why stuff isn't quite as relegated as you would normally expect).
I'm rather curious to see the Tau and Grey Knight lists to be perfectly honest.
Ask and you shall receive, this is again based of what i remember so any formations/squads that don't add up either is cause they cheated or i'm wrong.
He was rocking two large battlesuits, the forgeworld one and the other looking something akin to this (http://vignette4.wikia.nocookie.net/...20130603120203
Neither suit had drones attached. He had a crisis suit squad with his commander attached. He also had 4 marker light drone squads (4-5 drones each potentially 1 was a gun drone). He then also had another suit squad, might of been crisis but yeah that was his list.
So to sum up
2 Big Battlesuits
1 Crisis suit squad w/ commander attached
4 Markerlight drone squads (4-5 drones one might of been a gun drone)
1 other troop unit, likely a crisis suit
He rocked 2 Dreadknights with the massive swords and psy cannons
1 Chapter master, had a sword and thunder hammer
1 Grey Knight Termnator Squad (1 dual swords and storm bolter, One sword and psy cannon, 2 storm bolter and sword and 1 Sargent with storm bolter and sword)
He had a small list.
So thats the best my memory can do, idk if any of those combos are wrong or just right up illeagle but most of what i wrote is what they had barring the few exceptions where i wasn't sure of exact details.