I'm curious to know how much data the root namespace servers put out in terms of gbps, but this doesn't seem to be public information.
Select a root server at the bottom. Some, but not all, have a "statistics" link. Seems to be stated in qps and message size distribution, but you should be able to derive traffic volume from that.
This FAQ is illuminating: https://www.verisign.com/en_US/domain-names/internet-resolut...
The servers themselves are ordinary 1 RU physical rack mount servers with 1 Gbps or 10 Gbps Ethernet. Nothing special. I'm guessing that most of the load isn't from the root, e.g.: "j.root-servers.net", but from hosting the authoritative DNS servers for .com and .net (b.gtld-servers.net) on the same box. That would surely have more traffic and much more data.
https://github.com/rssac-caucus/RSSAC002-data
You can also click on the "RSSAC" button on root-servers.org to get the YAML straight from the root server operators themselves.
Most of the root server operators have anycast instances deployed in organizations that host the servers for them. So there's not an easy way to measure bandwidth utilization because many root server anycast instances are hosted in organizations that may not, or could not, report that bandwidth utilization. Look at the map on root-servers.org to see how dispersed around the world these things are.