So, some points to make on tuning theory - The 4.0L Hot V platform has some range on turbo pressures. This is the primary source of more HP/TQ for the aftermarket tuners. Stage one tunes usually only take a couple of tenths bar increase to produce about 50/60 extra hp. But, by doing so, they're also increasing cylinder pressures and temperatures.
When they go stage two, they're increasing boost pressure even more, and subsequently are likely needing to retard some timing too to keep cylinder pressures/temps in control. I'm saying it this way BC each tune house views its tune strategy as intellectual property and are not revealing what they do, but having tuned cars for over ten years now, there aren't any places they can go that's some big secret that has gotten past the rest of us. So, I'm pretty sure I can tell where your tuner is coming from.
Back to cylinder pressures, any amount of forced induction greatly increases cylinder pressures, and subsequently cylinder temperatures. If the tune strategy takes you up and away from factory specs, then the factory engineered spark plug range is also going to likely fall out of its useful high heat range. A spark plug used in a tune that has significantly more cylinder temps and pressures, begins to fail at the maximum pressures and temps and begins to contribute to spontaneous combustion (could even be pre-ignition, but that would be rare). Spontaneous combustion (commonly known as knock) is kryptonite to the engine and performance. Every tuner's job is to "beat down" spark knock. The ECU will do it if the tuner doesn't. but that would be a bad tune in any tuner's (or engineer's) eyes. We call those tuners "tooners" and, yes, they're out there.
When a tuner wants to go to a colder plug it's telling me that he's maxing out the timing as far as it can go with the increased boost, and with a colder plug he can hold a degree or two of timing in PE that he'd have to sacrifice with the hotter, factory heat range and get rid of some sporadic knock. Every good tuner deals with this. I've done a few NA converted engines to FI and routinely go two ranges colder on plugs with even modest 6 to 8 psi boost.
So back to the question about spark plugs. Plug manufacturers all use their own numbering systems to determine heat range. Some go up in #s for colder plugs, some go down in #s for cold range. I'll show two of the most common plugs below and how their numbering differs. So, bottom line, you need to know what plug you want to use and what's in your car, then you can pick the plug number you need to go one range colder. BTW, I think the GT series is using Bosch with a heat range 6, but not absolutely sure. If that's the case, Bosch goes down in #s for colder, so you want a 5.
Here is a typical Bosch range (not the actual plug in the GT, just a Bosch example)
With Bosch, they change the number in the middle of the part # to change heat range. So, if you had a Bosch FR6DC plug and wanted to do one step colder, it would be a FR5DC.
If NGK, they go in the opposite direction.
Here if your plug number is BKR6E-11, moving to a BKR7E-11 would give you one step colder plug.
One final comment on spark plug ranges - on the other end of the spectrum, cold starting - As the plug gets colder and colder in heat range (away from the factory spec) cold starts get choppier and choppier. So there aren't any concerns here, if you switched to a colder than factory plug and you live in Anchorage and you got up one January morning and the car was hard to start, idled terribly, and took forever to even out, then that is a consequence of going to a colder plug, but for the most part, one step colder isn't going to be dramatic on the cold start, especially if you live somewhere like I do, where the worst the cold start is going to be is about 30*/40*F.
Sorry for the length here.
EDIT:
Here is one more graphic I found that better shows a few plug manufacturers and how their numbering systems denote hot and cold ranges:
