The cheap "USB-CAN converters" are mostly ELM327 based.
IIRC the ELM 327 CAN interface are not full CAN. They are OBD diagnostics interfaces. The ELM 327 is based on a microcontroller that filters data from the vehicle CANbus.
So for monitoring a car, they can do the basics.
The next step up from the ELM327 is the newer STN1110 OBD interface. It's faster than the ELM327 and cheaper.
But still it is very focused on vehicle applications.
However if you want to monitor "any" (not strictly vehicle) CAN bus (industrial applications etc), you'd probably be better off (in my opinion) with the raw power of an SPI to CAN interface as the MCP2515 together with a CAN transceiver IC.
This MCP2515 interface is already supported in Linux under socketcan. You "just" need the hardware. Basically it's two IC's and some added components. It could be added to the Gert-board? Or it could be made into a small plug-on board for the GPIO connector so e.g. it could be easily mounted in a car for logging.