As I wrote in a YouTube comment and as others have mentioned you should not be setting the default gateway on the IB network.
It's not only not "best-practice" but will cause networking problems. I am not actually sure how you can still access the rest of the network with both PCs having the lowest metric route being via each other, but I am sure its some RFC breaking Windows magic if it does work.
If you communicate between the two machines using their IPoIB addresses then you just don't set the default gateway or touch any metrics and it will work.
If you want to be able use their LAN IP addresses and have it routed over IB (or just not worry about what IP is discovered using Windows's automatic discovery), then that's also possible. Again, don't set the default gateway or touch any metrics. Give the two machines static IPs (either manually or via DHCP). Add persistent routes between the machines for their respective LAN IP addresses.
Based on your example configuration and a Google search, using an Administrator Command Prompt, if we assume that the LAN IP for the NAS and Gaming PC are 192.168.0.20 and 192.168.0.21 respectively:
On the NAS-PC (192.168.0.20/192.168.1.20) run "route -p add 192.168.0.21 255.255.255.255 192.168.1.21"
On the Gaming-PC (192.168.0.21/192.168.1.21) run "route -p add 192.168.0.20 255.255.255.255 192.168.1.20"
This may be possible through the Windows GUI but again, I do not use Windows so cannot confirm this.
In my opinion, the configuration you have given is fundamentally flawed, does not achieve what it sets out to do and is misinforming people who do not understand how networking, routing and IP work. I hope you can take this feedback, test it and make the appropriate corrections.