Alright, after some digging I found that that the ACPI address of my dgpu is
\_SB_.PCI0.RP01.PXSX

Then I used https://github.com/mkottman/acpi_call to execute
\_SB_.PCI0.RP01.PXSX._PR3 as you suggested, but it reported an error
(NOT_FOUND). The examples in that repo suggest that _PS3 or _OFF might also
work. _PS3 gave NOT_FOUND too, but I could execute _OFF without error. In other
examples there are more calls necessary though, e.g. NVOP or _DSM, but how can
I know what's needed?

To power back on, _ON seems to be working.

So, what do you suggest I should try with these methods exactly?