It is already covered in the rules,
from 1.1.13 Unit Datasheets, highlighted the relevant part

I don't think an FAQ about power fields can do any harm: in particular, I have seen people roll for them at different points around the start of the game (not that I care). Like:
1. roll them all first thing when discussing the list with the opponent
2. roll for each gargant's fields as it is deployed
3. roll them all before making the first strategy roll

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4. Roll them the first time an enemy unit shoots at a Gargant, because the player totally forgot about them...

I have always played that they are rolled the first time the gargant needs them. Everything else, the randomness is rolled when it is used. The same would be true of the gargants. Because they get a random number of shields, but cannot recover them, there's no reason for them to get rerolled in later turns, but it's definitely better if they aren't rolled until they're needed. That way, neither player can plan from the beginning to focus their firepower around what turns out to be the weaker gargants and it's more orky to find out the strength of it when they actually come under fire.

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Back on track! Got a 4K game against a EpicUK Tyranid list on Saturday. This is the list I'm taking to it:

Morks Mega-Manglerz, 3995 POINTS
Gargant Bigmob (3.2.2 - Developmental)
==================================================

GARGANT [700]
(Kustom) Wyrdboy Tower, (Kustom) Power Fields, Snappa, Ripper Fist, Grot Guided Missile

GARGANT [625]
(Kustom) Power Fields, (Kustom) Reinforced Boiler, Snappa, 2 Gatling Kannon

GARGANT [625]
(Kustom) Power Fields, (Kustom) Reinforced Boiler, Ripper Fist, 2 Gatling Kannon

SUPA STOMPA [350]
3 Soopagun, (Kustom) Extra Armour Bitz

SUPA STOMPA [350]
3 Soopagun, (Kustom) Extra Armour Bitz

FIGHTA-BOMMERS [200]
4 Fighta-Bommers

FIGHTA-BOMMERS [200]
4 Fighta-Bommers

FIGHTA-BOMMERS [200]
4 Fighta-Bommers

LOOTA WARBAND (NORMAL) [150]
6 Boyz, 2 Grotz, Killa Kan

LOOTA WARBAND (NORMAL) [150]
6 Boyz, 2 Grotz, Killa Kan

STOMPA MOB (BIG) [445]
7 Stompa, Flak Wagon

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Interesting list, can't wait to see how it goes.

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That's the exact list I used a little while ago against a Siam-Hann player. The match was transcribed for a Batrep but I haven't gotten around to writing it up and posting it. The game went to the Eldar player 3rd turn with a 3:1 victory. It was pretty close up until the end and came down to a few key engagements. It was quite the struggle to play effectively against such a mobile force. The Eldar dictated the match from the get go in terms of engagements. The Ork strengths were Air superiority and the Gargants being very tough. It was quite a challenging game, constantly had to make the most of unfavorable situations. Saim-Hann has soo many scout options it was really tough securing victory Points being forced into engagements and herded around.

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A player in our group has played a couple of games with the list now, most of the time he spams flyers and the auxialliary units, coupled with 2 gargants, both with boilers and double flakwaggon for highspeed objective grabbing (as the waggons only have to be in dc x 5cm coherency to the gargant).
This tactic can easily be exploited, of course, but he tends to secure them pretty well. Plus the WE can block LOS if needed.
Overall my opinion, after facing them a couple of times and watching another handfull games, the list seems to be pretty exploitable. If you stick to the classic stompy units, the army is slow, devastating and colourful (as it should be), but you can easily change it into "heavy walker and flyer ghazgkull" (as it should not be).

I even tend to not play against the Gargant Big Mob anymore as I am pretty fed up with it (the ridicoulous special rules for rallying are just the icing on the cake). And thats not whining because I lost some games, it just feels wrong and, at least in my opinion, I dont enjoy the games against them.

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Could you be more specific about what builds that are troublesome? Exactly how are they exploitable?

And the flakwagons have to be within 5cm of the gargant not 8DCx5cm...

Thanks for your input!

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Any flakwagons deployed like that are asking to be engaged (or just shot up for easy BMs). It does seem a bit odd to have them in the same formation but they are orks after all.

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Hi, how does the list look? I have tried to build lists with only 2 gargants but then you only get 6 support formations so its kind of hard to spam flyers and auxilliary troops for me at least. I usually end up with 3 gargants and then you have the issue with enought points instead
Also if you know about the tactic, taking those 4 flakwagons out should not be that of a big problem, right?

I've played with my gargant big mob force 5 times and have a 2:3 (win:loss) record (though never using the same composition twice). I've tried the added flak wagons to the gargant and it does work pretty well initially. They aren't that easy to kill since you can block LIS with the Gargant and the large cohesion allowance for war engines does make for a good last turn objective grab. Though one game pulling this on someone and your opponent should be able to shut it down susequently. That said it is still a tough list to face.
@Largo_W would be very interested in seeing the list your opponent was using. Fairly sure I've tried something similar. This is what I used:
Incompertus, 2990 POINTS 
Gargant Bigmob (3.2.2 - Developmental) 
================================================== 

GARGANT [720] 
2 Flak Wagon, (Kustom) Reinforced Boiler, (Kustom) Head of Mork/Gork, 2 Mega Lobba, Gatling Kannon

GARGANT [670] 
2 Flak Wagon, (Kustom) Reinforced Boiler, 2 Gatling Kannon, Ripper Fist

FIGHTA-BOMMERS [150] 
3 Fighta-Bommers

FIGHTA-BOMMERS [150] 
3 Fighta-Bommers

FIGHTA-BOMMERS [150] 
3 Fighta-Bommers

FIGHTA-BOMMERS [150] 
3 Fighta-Bommers

LOOTA WARBAND (NORMAL) [200] 
6 Boyz, 2 Grotz, Big Gunz, Oddboy

LOOTA WARBAND (NORMAL) [200] 
6 Boyz, 2 Grotz, Big Gunz, Oddboy

LOOTA WARBAND (NORMAL) [200] 
6 Boyz, 2 Grotz, Big Gunz, Oddboy

LOOTA WARBAND (NORMAL) [200] 
6 Boyz, 2 Grotz, Big Gunz, Oddboy

LOOTA WARBAND (NORMAL) [200] 
6 Boyz, 2 Grotz, Oddboy, Big Gunz

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