My immediate thought is that Gus was an orphan in the Outer Rim. There is a planet called Lahn, that may not be his planet of origin, but is the origin of his story.

He was orphaned at a young age on Lahn, and Guss was picked up by a mysterious hermit. This hermit cared for Guss throughout his childhood, teaching him to live off the land, survive, and find his way in wild. He instilled some...training, so to speak, into Guss, the same way that Mr. Miyagi initially trained Daniel in the Karate Kid. He was taught form, but never technique.

Eventually, a ship came, and Guss was returned to his ancestral homeworld of Devaron where he reintegrates with society and eventually married and had children. Between his upbringing and cultural dispostions, he found his strengths in traveling and working to provide for his family - letting his wife, Sevu, handle family matters at home.

Tython is not a place that Guss has visited - but has heard of - and he has a nagging feeling it is somewhere he wants to visit.

The Galactic Republic has been a primary employer of Guss since he reintegrated with society, as he largely works as a contractor and guide.

At the time of this, Guss would be nearing 44 years old. Born before the Galactic War began (if that time works from what I see you wrote), he intermittently worked across the galaxy for the Galactic Republic to explore and locate suitable bases, find missing peoples and military units tagged as MIA, and acting as a guide for Special Forces on backwater and wild worlds. The one thing he could never shake, though, was the violence. He wasn't good at it. But he did what was needed to get paid and keep on.

Now, 5 years after the war, his direct purpose has been unclear as he searches for new opportunities.
Having served as a guide/wayfinder etc for Republic forces during the war, you would have also participated in at least one covert operation for the SIS.

Over the course of years, you have helped the SIS (knowingly) twice:

About 20 years ago, helping extract Republic loyalists from Belkadan (swamplands).

About 6 years ago, helping rescue an asset (one Baron Ghen) from Serenno, which the Sith had just conquered (rainforest).

The first mission was a success.

The second… was a disaster. Ghen was assassinated by a team of Imperial Agents. Your team was almost wiped out.

However, as a result of this history, you know of a way to contact the SIS if necessary…
The Belkadan mission went almost flawlessly - Guss would have been absolutely stunned by the lack of bad things that happened. Even some of his normal contracts with the normal service members and private comanies/parties alwags had a snag.

The Serenno mission - that was a nightmare. From the absolute start. Local fauna, specifically Spine-wolves, attacked the SIS party during one overnight. The lack of fire due to a cold camp necessity drew them in. Guss drove the beasts back with his Air Rifle and sheer presence. Some would say it was force like, when he locked eyes with the last spine-wolf before it tucked tail and dashed back into the forest. Unfortunately, this left one dead and one badly injured. Unable to travel with the wounded, and equally unable to leave someone to guard them - Gus injected them with enough tranquilizer to let them sleep through the end.

Then, as the team had just begun their extraction - they immediately found this extraction was a trap. Ghen was killed, but not in the first shot. Guss had thought his action was prudent, as he spun and absorbed the first blaster shot meant for the Baron. It was instinct. As he fell, the second shot killed Ghen cleanly. Guss was carried into the surrounding woods by the remaining agent, one of the only remaining soldier types on the team. As they approached the woods while being pursued heavily, Guss thought it a fever dream. Flashes of the spine-wolves in the spaces between trees. Then the screams, as the Imperial Agents were torn apart, scattered in the wood line.

Guss and the remaining SIS agent melted into the woods, finding their hidden ship and escaping.

I would think this other Agent who survived might be the contact for Guss to talk to the SIS.
I was thinking it's one, very specific way to find his contact.

Physically going to Plexis and finding a specific tavern, where he has to ask about finding anyone who can teach him Serenno music
I feel like this SIS agent was also wounded and has taken on an information role in the SIS instead of hired muscle. Monitoring spice gangs and any potential business they have with the Sith Empire.

Wealth: Accumulation of funds to send home.

Obligations:
Family - 30 (+15 Starting, plus +10 XP, +1000 Starting Credits)
Guss holds a strong affection for his family. While he didn't grow up on Devaron, he loves his home, and more importantly, the family he has started there.

Morality:
Emotional Strength - Love
Emotional Weakness - Hatred