Played against this list today.
Gorgrim took the following (from memory):
Great Gargant + free Kustom Upgrade (Gattling Kannon) + 50pt Big Kustom Upgrade (Gattling Kannon) + Mekboy Big Boss
Gargant + free Kustom Upgrade (Gattling Kannon)
Supa Stompa + free Kustom Upgrade (Gattling Kannon)
Supa Stompa + free Kustom Upgrade (Gattling Kannon)
Supa Stompa + free Kustom Upgrade (Gattling Kannon)
Loota Warband + Oddboy with Kustom Upgrade for free (Megalobba)
Loota Warband
I used the EpicUK Nid list.
My first impressions of the Gargant list is that the Gattling Kannon makes this
Ork army incredibly shooty!
Ork War Engines were Advancing (single move and shoot)
very often. This made the Gargant army feel less
Orky to me. The number of MW barrages and good quality shooting was fearsome. And then when we got to the Engagements, the Gattling Kannon War Engines just got even better with all the extra FF attacks. The Orks did not make a single CC Engagement all game even when in position to do so (partly because it would have benefited me more but mostly because their FF was just so much better).
Now I'm not so worried about the outcome of the game (a 3:0 to the Gargants but we're both using new armies) as to how the Ork army played. Gorgrim played them very well and made the best of the armies strengths. Often, I just didn't really feel like I was playing against Orks though.
We did discover that broken Supa Stompas are hard work to rally (a possible good use for Stompa upgrades to build the formation up to help them rally).
The army is slow... (pretty obvious but Gorgrim has been playing Speed Freeks and Eldar recently which makes the Gargats a rude shock).
The idea of being able to teleport (Planetfall) a Great Gargant into the middle of 3 objectives (right into the opponents face after the Orbital Barrage has prepared the landing site) with no chance for taking BM's as per a normal teleport and then retaining with a FF unit of doom is an amazing prospect.
My first thoughts are to nerf the Gattling Kannon in some way. It is 50pts (or free

) which is not cheap but it's effects on the Ork War Engines is huge. 6 good quality shots just doesn't feel very Orky. And then factor in the bonuses for FF and the Gattling Kannon is a no brainer.
As for ideas...
4x AP5+/AT6+ with +1 FF?
6x AP5+/AT5+?
Remove the free Kustom Upgrade?
As I said, this is only 1 game so bear that in mind. It's too early for me to draw serious conclusions. These are just my first impressions. I'm really looking forward to more games against this list.