Would a fibre optical isolator or ethernet switch do the same thing as the Gigafoil?
Yes.
As with any configuration, it depends on the components used and their set-up.
For a lot less expenditure, installing a run of optical fiber in-between your "upstream" router & music server and "downstream" network bridge or streamer, and connected at each end with FMCs (Fiber Media Convertors) using optical transceivers will provide the exactly the same functionality as a GigaFoil, and notably improve audio quality in the same manner (from a Physics-based "transfer function" perspective).
As shown:
You can use generic (e.g., TP-Link, Netgear, etc) FMCs but the Sonore OpticalModule Deluxe FMC (OMD) is
much better as it has LC/LC optical fiber SFP cage, and a much better master (word) clock than the generic FMCs, as well as subsystems and internal configuration designed to lower the impact of various classes of "digital-derived" noise, and it's affordable at $499. It was designed by the designer of EtherREGEN, John Swenson, who worked as a professional Ethernet EE at Broadcom and Cisco his entire career, and designed OMD.
Link to OM-Deluxe:
Sonore - opticalModule Deluxe
You could use a generic FMC at the upstream end and the OM-D at the downstream end, or for better performance, use an OM-D at both ends, which would still be 1/2 the cost of GigaFoil. Then just run Ethernet cable to your network bridge or streamer.
A lot folks also use an EtherREGEN Ethernet switch, which also has an SFP cage in it and provides the same FMC functionality, but provides for more flexibility for "networked audio" set-ups. Here's a pic showing that setup, where OM is the upstream FMC and ER is the downstream FMC.
That's set-up I've used for over 5 years and it's worked really well.
Note: You don't have to use ER in this setup, though, you could just use another OM-D.
The key point is you don't have to use a GigaFOIL to obtain the benefits of optical networking, there are a number of other solutions designed specifically for high-end audio that provide the
exactly the same functionality for less expenditure.
I have a lot of experience using optical networking for high-end audio setup, so if you have any other questions, just let me know. Happy to help out any way I can.
Cheers.