The 'Guitar expansion base board' is known to only include 10/100 Mbits/sec Ethernet according to your product page.
In the block diagram 'SR8201' is written. With this name there exists an USB-to-Ethernet adapter but the block diagram shows a connection between SR8201 PHY and S500's internal 'MAC' (according to the specs the S500 is '10/100Base Ethernet MAC equipped'). So this is native Fast Ethernet on the expansion board?
And the only way to get reasonable speeds with GBit Ethernet is to use an USB3-Ethernet adapter on the Micro USB 3.0 port the board provides?
The S500 include a 10/100Mbit/sec Ethernet MAC,and the Ethernet PHY(SR8201)is used in the expansion base board.So the Guitar support 10/100Mbit/sec Ethernet.if you want to use the Gbit Ethernet,you must to use a USB3-Ethernet adapter on the Micro USB3.0 port or wait for the next new product.