Can anybody recommend a decent gigabit usb adaptor?

It doesn’t reach full GE, just about one third of it (see here).
Either iperf (search the forum, there are dozens of posts how to set it up) or a quick test via FTP with one big .iso. Either way, bitrates higher than FE for simply playing a video file are only needed in a very small number of cases. Heck, even the UHD Blu-ray remux of John Wick: Chapter 2 with its insane bitrate plays smoothly on the Vero 4K mounted via NFS.