What you're describing sounds like the process for adding NVMe support to a board that shipped with a PCIe M.2 slot but no NVMe driver in the firmware. There were quite a few of those circa Intel's Z97 chipset, and motherboard vendors were inconsistent about when or whether they shipped firmware updates to add NVMe support. A few early NVMe SSDs included a PCI Option ROM to provide NVMe capability on systems that didn't already have it. For any other NVMe SSD, the firmware needed to be modified to add a UEFI DXE driver for NVMe to enable booting.

There were very few consumer PCIe SSDs sold before the adoption of NVMe, and they were almost exclusively sold to OEMs rather than as retail products. A few like the Samsung XP941 and SM951 were available through grey-market retailers for about a year before Samsung launched the 950 PRO as an official retail product, using NVMe (with an Option ROM). (Note: the SM951 existed in both AHCI and NVMe variants, so some of those grey-market retail sales were of drives that were essentially 950 PROs with slightly older flash).

It's not entirely impossible that Gigabyte shipped boards that couldn't boot from AHCI PCIe SSDs. But it would be difficult for that to be a driver issue. I'd expect those boards to also have difficulty booting from an add-in SATA controller card, because the difficulty booting from AHCI SSDs would have to come not from a lack of an AHCI driver but from a misconfiguration preventing that driver from being used with anything other than the SATA controller built-in to the motherboard.

How sure are you that you didn't miss the brief era of AHCI SSDs entirely and are simply remembering early NVMe teething issues?