Welcome to the forum. There are quite a few former GS owners riding Super Teneres.
Necessary farkles depends solely upon how you ride and what level of comfort/protection you desire. Nothing is a given or must have.
If you take the bike off road, protecting the cast aluminum sump and side mounted radiator, (and preventing the fan from being pushed into the radiator, which can both damage the radiator, and fry the fan when it can't spin), is a good idea. That means some form of skid plate and crash bars. There are a variety of protection levels out there in the market place. Only three skid plates don't mount to the somewhat fragile cast aluminum sump. Ride On ADV, Rumbux and ACD. ACD has had some delivery issues lately, so not sure if they are still a player. Rumbux is order on demand from South Africa, so there is a wait time involved, but it's a unified system of upper and lower crash bars and a skid plate bolted to that frame work, Ride On makes the hands down most bomb proof skid plate and you get what you pay for.
Disclaimer - I'm the US importer for Rumbux Adventure Products.
After that the usual common items are out there. Windscreens, wind deflectors, headlight guards, seats, ECU Flashes, aux lights, etc. Options abound.
Probably the most common items beyond the skid plate/crash bars is a aux fuse block of some kind if you're going to add electrical stuff. No CANBUS to worry about, so fairly simply. Eastern Beaver makes the most popular plug and play options. (PC-8). Plenty of roll your own stuff too. The bike comes with pig tails for oem accessory lights, though the '14 may be different as it uses an integrated heated grip control via the ECU. Not sure if they kept the pigtail for the aux lights.
Some of the GS stuff may transfer over, depending on your choices there. I run Zega Pros on my S10. Not being a shaped pannier, if you had Zegas on the GS, you might be able to just buy the mounting system and keep the same panniers. Other brands may or may not offer similar potential.
