TL;DR
Gigabyte is refusing to support a product they know to be problematic. This is as of the time of writing still in production with the same components. We haven't asked Gigabyte for a public statement.
Most likely problem - Main FETs
To start, a quote from Aristeidis Mpitziopoulos', which in this article will be referred to as Aris, review on Techpowerup about this very unit below.
This is something alarming on it's own already. The main FETs on the power supply explode after having handled a heavy load. They didn't fail during the actual OPP/OCP testing itself, but a few minutes after when the unit was turned on again. But this is sadly only the component that seems to cause most of this issues...
Gigabyte handling this finding - we tested it and it's fine
From here, Aris attempted to send his findings to Gigabyte. They chose to not recall the unit they sent him and tested around 10 units internally in their RMA division. From here, no issues were found and this was reported back to Aris. This is in every way a flawed way of testing. More about this can be found in the video here:
Gigabyte handling it's consumers - refusing support
Gigabyte has had many reviews on sites like NewEgg that claim DOA units, extreme coilwhine, killed components and much more. Normally I'm not one to react off consumer reviews because of their flawed nature, but when almost a majority of users has failures and there's a ground of suspicion what is the problem it becomes a different story. A few quotes below
These are a few of the more elaborate reviews. As of the time of writing, 142 out of the 242 reviews on the PSU are 1 star or "egg", all coming down to either extreme coil whine, system instability or the unit dying with no way of returning it without including the 3000 series card they bought it with. Even if it's understandable that Newegg/Gigabyte doesn't want a scalper to buy this and attempt to return the PSU, with a genuine failure this is an outright stupid thing to do.
Source: https://www.newegg.com/gigabyte-gp-p750gm-750w/p/N82E16817233029?Description=gigabyte psu&cm_re=gigabyte_psu-_-17-233-029-_-Product
What now? - Forced recall and public apology
Honestly, the best thing Gigabyte can do right now is putting on a forced recall on the unit. There are no exact numbers out there on the actual failure rate of the unit, but this isn't down to an early production sample unit or the occasional "lemon" you get with mass production. This is a widespread issue that can only be solved by using different parts in the PSU. They can either choose to take it off the market, issue a full recall and give people their money back, or introduce a revision/newer version of the product with higher end FETs from for example Infineon, with likely also a lower OCP/OPP setting.