GTO's all have LS motors, LS1 in 2004 and LS2 in 2005/2006. Fbodies got the LS1 in 98.......
The reason people use the 5.3 is cost, LS motors are aluminum block where the 5.3 is iron. Also, SOME people like to use a good iron block is they plan to make HUGE boost levels and are worried about breaking the LS alumimum blocks. But a good aluminum block built with forged pistons and rotating assembly should be good up to 1500hp anyway...... Maybe more.
Not to start an argument or pissing contest but I did say "pre-LS" cars, like all the 60's and 70's muscle cars Such as GTO, Chevelle, et al. My avatar is a 2000 TA. Its an all aluminum LS1 and I am well aware of the LS's use in pretty much everything since the 97 'Vette including Holden's variants of performance cars like the UTE, Commodore, and the Monaro (04-06 GTO).
Most of the LS family since 1997 (4.8, 5.3, 5.7, 6.0, 6.2, 7.0, & 8.1) were/is (depending on variant) available in aluminum and/or iron block (LM7 5.3, LQ4 6.0, LQ9 ** 6.0).
The 2015-2020 K2XX SUV "LT" motors (L83 - 5.3 & L86 6.2) are additional variants of the "LS family" just direct injection, but still interchangeable with some earlier LS parts.
The 6SPD transmissions on the newer trucks seem to make it ~150k miles and then the converter conks out and takes out the trans if you don't catch it quickly. Lots of documentation on the service side about this, although it isn't nearly as widespread as the 8SPD issues since it's more of a high mileage problem. I'd put TC upgrade on my "maintenance" list lol.
RedChevy is absolutely correct about the 8 Speed autos. Not a good job by GM on these trans. Look for 6 speeds. 2017 seems to be the year to go (I got lucky) where there are still a lot of 6-speeds in the market and they worked out the kinks of the 1st and 2nd years of the K2XX generation of SUVs.
I just worked with Circle D for a converter. Triple disc lockup, 3000 stall. A couple of GM transmission buffs I've interacted with on TikTok all recommend putting in a thermostatic bypass valve for the 6L80's. The thermostat from the factory raises the temp in the trans really fast for emissions and drivability purposes and holds it at the 185 ~195 temp. Everyone of those people said high temps kills the trans and that 195 number is close to the breaking point. The thermostatic bypass valve should pull those temps under 180 or better and makes all the difference in longevity and performance. They also recommended the Circle D TC.
Also if you have a heavy foot, they recommend a larger diameter aluminum driveshaft. They stated the factory ones don't hold for long after crossing 90mph.