Depending on how recent your pfSense install is, you may need to use the python 3.7 version of the plugin. If you are running 2.4.5, you probably want telegraf_gateways-3.7.py. They both output he same data in the same format.
All I did was copy telegraf_gateways-3.7.py to /usr/local/bin and rename it to telegraf_gateways.py
Does /usr/local/bin/python2.7 exist on your pfSense system? If so, use this telegraf_gateways-2.7.py
Does /usr/local/bin/python3.7 exist on your pfSense system? If so, use this telegraf_gateways-3.7.py
If you have both 2.7 and 3.7 on your system, use 3.7
Thanks to this thread, user bigjohns97 for following up and fastjack for the dev effort.
I found a plugin to get IP, MAC, IF Name and Status. On this thread a user posts some go code for a telegraf plugin. I fired up a FreeBSD 11 ami on amazon, installed go and compiled a binary version of the code. It's worked as expected but my queries and formatting could use some help.
I saved the code as telegraf_netifinfo_plugin.go and compiled with the following commands:
setenv CGO_ENABLED 0
setenv GOOS freebsd
setenv GOARCH amd64
go build -o telegraf_netifinfo_plugin