Would a 0.7 teleconverter built-in to an adapter work? 18mm/26.7mm=0.674. The 300 F2.8 lens would become 202mm F1.9? That would still be useful if that's how the optics work out.
There are those kinds of adapters available. although to my knowledge, not E to G. There are some optical compromises involved, as with all TCs.
My overview of such issues in general is
Mixing and matching sensor format with lens coverage published at Electronic Imaging 2016, but let's get specific about E and G and why that's not a happy path to follow. At least not happy for focusing to infinity and having the lens focus and aperture operational...
Adapters with fractional magnification are called
focal reducers. They're quite common for using FF lenses on APS-C. They reduce the focal length by the stated factor, and since the aperture size is unaffected, that also reduces the f/number. However, reducing the focal length shortens the rear focus distance and reduces the coverage circle. Reducing the coverage circle tends to increase MTF resolution, which helps when putting a FF lens on an APS-C body but not when an E/FE lens goes on a G body.
So-called "
glass adapters" are even more common. They are basically teleconverters with just enough magnification to extend the rear focus enough to hit infinity using a body with a longer flange distance than the lens was intended for. Typical magnification is around 1.2X, and you'll most often see them for things like using a Minolta MD lens on a Sony A-mount (aka, Minolta AF-mount) body.
To use a Sony E (really FE, the FF variant of E mount) lens on a GFX, you need a glass adapter to extend the rear focus from 18mm to around the 26.7mm flange distance of the G mount + whatever thickness the adapter adds. Realistically, you are probably talking about 30mm or so, which means increasing the rear focus by a factor of 30/18, or about 1.67X. That's not hard to do optically, although that's also not a standard teleconverter magnification; one might substitute a 2X. Using a 1.67X, you would be converting a 50mm f/2 lens into an 83mm f/3.3.
As less-than-exciting as that sounds for wide-angle lenses, there is also the issue that native Sony FE lenses depend on an electrical connection and communication with the body. Contrary to common wisdom,
Sony does NOT make their lens protocol freely available, but it has been reasonably well reverse-engineered.
Here is an brief overview with links to what is known about the Sony E/FE protocol. You would need the adapter to contain a processor that could translate between E/FE and G electrical interfaces and protocols.
The G lens interface protocol is even less openly understood, with very preliminary work like
protocol sniffing toward reverse engineering still going on.
In other words,
making a high-functioning E/FE to G adapter is not easy, requiring both teleconverter optics and a microcontroller to translate between two lens protocols, neither of which is openly documented. That's probably a hard
NO.
Incidentally, the Hasselblad XCD has an 18.14mm flange distance, so you might get away without the glass adapter, although most FF lenses don't quite cover the 55mm diameter image circle implied by a 44x33mm sensor. In any case, the protocol conversion issue would still be there for sticking an E/FE lens on a Hasselblad...
