Assuming you use the gauge for things like oil temperature, AFR, etc.:
if it doesn't work it just means that it isn't a well-designed product.
The way the OBD-II protocol works is that each device first sends a request for certain data (PID request),
and then the car responds with that data. It is a high-level protocol implemented over lower-level protocols,
such as the CAN bus. The OBD-II connector has the pins for the CAN bus (more on that later).
OBD-II is not a very efficient protocol. The biggest benefit is that it's standardized, and widely supported.
It's possible that when there are multiple devices requesting info at the same time using this protocol
they start to interfere with each other, up to the point of not functioning at all.
A purpose-built digital gauge for BRZ/FRS/86 could instead read the data from the CAN pins of the OBD-II
connector without using the OBD-II protocol. The car broadcasts a lot of data
all the time
(see some info
here), without the need for the peripheral device to make any requests.
The downside is that it's not standardized.
If a digital gauge designed for BRZ/FRZ/86 can't just listen to those, it's not a very well designed gauge.
From the manufacturer's perspective, "it works in most cases" and they've either never tested
interoperability with other peripheral devices; or not prioritized that.