And a little straightening out as regards what drives generators...

And a little straightening out as regards what drives generators [and what does not] would not go astray, while we are at it. I'll stick to engines already mentioned here. There are dozens of other possible options from many other manufacturers:
Gas turbines:
JT8D - Built by Pratt & Whitney, these engines were used on early 737, 727 and other now retired aircraft. Aeroderivatives of the JT8D include the FT-4 and FT8/ GG8 for use in power generation and other industrial applications. An old technology, low bypass, noisy, thirsty beast pretty much obsolete in airline service. You can still buy these new, now built by Mitsubishi Heavy Industries as a generator package, if you want some rugged classic machinery. 33Mw output, about US$21.5M list price new, just as a rough number to work with. Probably cheaper on Temu.
There are three pretty old FT8s still being used for peak grid loads at the Tamar Valley Power Station in Tasmania.
CFM56 -Built by a consortium including GE in the US and various European partners and used on later 737s, some A320s, USAF KC-135 aerial refuelling tankers and many other aircraft. Many different variants. AFAIK the CFM-56 does not have an aeroderivative version for non aviation use. Sales started slowly in the late 1970s and they almost pulled the plug on the whole US-Europe project after 5 years of no sales, but it is now the most popular engine in airline service. Too busy building them for aircraft these days to be bothered developing a derivative, I should think.
CF-6 - Built by GE, about three times the thrust output of a CFM56. These are used on large aircraft. e.g. Qantas used them on about the last dozen of their 747-400s [all now retired], so about 50 engines, and they are still used on all of Qantas' A330 fleet, 26 aircraft remaining in the fleet, so say around another 55 engines including spares.
The LM2500 is derived from the CF-6, not the CFM-56. A few versions are available with different power outputs, but nominally around 34 Mw in simple cycle. There are over 1,000 of these in service in the ships of many navies, including the RAN, USN and RN, where high output and high reliability are very necessary. Many more of the LM2500 family of engines drive peaking power generators. These derivative engines used to be built by a division of GE. This has now been hived off from the main GE organisation into a stand-alone company, [somewhat confusingly] called GE Vernova. Like Boeing, and Qantas for that matter, GE is no longer the highly functional, vertically integrated company it used to be, having been beaten into the ground by over-rated management, mostly Jack Welch, who cut it to ribbons. But they still do gas turbines extremely well and have a vast amount of experience building them and keeping them running.

Reciprocating engines [Otto cycle**] running on natural gas.
https://www.jenbacher.com/en/gas-engines/type-6
Not a new or untried concept. Just something you wouldn't normally expect to see in a peaking power plant. Typically you'd see these in remote, off the grid locations where there might be multiple units in the power station of a large mine site or a town. So, yes, a number of quite small engines that can be switched in or out as demand changes over the course of the day would be useful there. In a peaking power situation, the grid operator will typically say "we need your whole 85 Mw and we want it pretty much now, or in five minutes from now, tops". Both simple cycle gas turbines and reciprocating engines can do that. Watch an A330 aircraft at Perth Airport start up, taxy out and depart. Five minutes from cold metal to takeoff thrust is the norm, same thing departing the next place, half a dozen times per day, every day. Aircraft don't have time in their tight schedules wait around.
A minor advantage of having multiple small reciprocating engines is that there can be a couple of them down for maintenance at any one time while still meeting full dispatch obligations. There are some disadvantages also, which will become apparent in October 2026.
STX have already made their choice. As no one in STX management is likely to be an expert in engine technology, I guess the choice they've made comes down to the advice they received and from where it originated.

** Otto cycle just means an engine having a spark plug for ignition, as distinct from a diesel, which does not.