Kolles will be familiar with the Gibson from running it in their last LMP1 chassis. The engine is available and relatively simple. Presumably it is available on a lease basis from Gibson, like it's little brother GK428. So it's appealing from that point of view.
In LMP1 days it apparently made about 660-670bhp according to this Rebellion story in Racecar Engineering
https://www.racecar-engineering.com/...ical-analysis/
As the only naturally aspirated, non-hybrid engine in the class, where it will surely be falling short is torque. Not such an issue in an 830kg LMP1. Rather less ideal in a 1030kg lard bucket Hypercar.