For those interested in the Victron 5kva Multiplus with the build in Gx, please remember the Gx only has one Can Profile and it comes pre-configured as a 500kbs BMS-Can Port. There is no way of selecting other baud rates.
You can not use this port to communicate with a Can Bus MPPT, the port is only for communication with a Can-BMS.
My friend @plonkster knows why it does this…
Perhaps it is because it is based on the NanoPi and does not have that many ports? Just my guess.
OK, simple enough. The Nanopi does not come with a CAN interface on the chip. So one had to be added, and the nature of what was added is the reason for this.
Let me expand a bit… as I always do 
These days chip makers make these really nice things called SOCs, System on Chip, it’s a single chip that has a CPU, some memory, a number of GPIOs, and some other peripherals as well. Many of these SOCs come with a CAN-controller on the chip. For example, the STM32F4 comes with a CAN-controller, while some of the others in the STM32 family do not.
Where the SOC comes with a CAN-controller, you only have to add a CAN-transceiver chip (a little 8-pin thingamabob, usually either an MCP2551 or an ISO1050, or something from that family).
So… some of the Venus platforms have a built-in CAN-controller, and some do not. For those who do not, such as the Nanopi used in the -GX inverters, an external one is added. For the Cerbo, one of the CAN-controllers is on the chip, and the other one is added on. The Cerbo and the Nanopi-based models use the same add-on chip.
The chip that is added on uses the slcan driver: CAN over serial. It is a chip that translates the CAN-communication to serial. You can find such chips at hobbyist places, for plugging directly into a laptop, etc. But this solution is limited in terms of speed and volume, and this is where things go a bit squiffy.
The chip does fine at the relatively low volumes of a CAN-bms battery, but not so well at the higher levels required by CAN-bus MPPTs. As a result, the slcan port is reserved for battery comms. On the Cerbo this has the result that only one of the ports can be used for MPPTs, and on the -GX inverters it means you have to use VE.Direct to the MPPTs, the CAN-bus is only for the battery.