Going through the list of Intro Hour posts since there are quite a few I haven’t done yet, and couldn’t resist this one.
The burn scars that cover the right side of Gunnar’s face, along with his upper torso, back, and arms are all evidence of the biggest betrayal of Gunnar’s life. He is also almost completely blind in his right eye due damage from being burned. Three years before Gunnar became the leader of the Veilguard, he had been dating another crow, Ronan. Viago hadn’t been thrilled about it, but all he did was warn Gunnar that he should not wholeheartedly trust Ronan. Gunnar, knowing Viago’s paranoid tendencies, mostly ignored anything Viago had to say about the relationship.
Ronan was a very ambitious Crow. Deadly with a blade, known for being a very efficient and skilled assassin despite not belonging to a higher ranking House. His ambition is what led to him betraying Gunnar. The Crows all knew Viago favored Gunnar long before Viago ever became the Fifth Talon. Ronan was turned down by Viago when he asked to join House de Riva, which led to Ronan trying to find out why Gunnar held such high favor with the Fifth Talon despite being the opposite of what Viago usually expected from anyone in House de Riva. His main tactic was making Gunnar fall for him, so he could get closer to not only Gunnar but also have more access to the Fifth Talon. However, it was the First Talon herself that eventually gave Ronan the information he needed to uncover the truth behind who Gunnar is and why Viago made him a member of House de Riva.
Viago never suspected the contract he received would lead to a setup. He gave it to Gunnar because it was supposed to be a simple contract for a mage who had been working with the Venatori, helping them acquire some artifacts and even helped them traffic slaves. Gunnar went to the location, only to end up ambushed because the mage and half a dozen Venatori knew he was coming. He managed to fight them all off, and as the mage tried to escape, they threw a fire grenade. The right side of his body, especially his upper half took the worst of the burns. He collapsed due to the sheer amount of pain he was in. Left for dead, Gunnar remained where he had fallen for two days until Viago found him and took him back home.
It wasn’t until a few weeks later that he learned the truth from Viago. Ronan tipped the mage off, hoping that Gunnar would be killed. Ronan’s motivation was because it would earn him high favor with the First Talon and could potentially get him into House Dellamorte. Gunnar was, of course, devastated to know he’d spent a year loving someone who never loved him back. He hadn’t had any idea Ronan’s affections were false. He also never got to confront Ronan because Viago, outraged that someone dared cross a member of House de Riva, absolutely snapped and took care of Ronan himself when he discovered he was behind Gunnar nearly being killed.
When Gunnar was betrayed by Solas, it brought up a lot of emotions he’d buried after Ronan’s betrayal. His anger and frustration boiled over, which led to him choosing to fight Solas and force him to be bound to the Veil.
To clarify a little about Caterina giving information to Ronan - Gunnar is technically the last of House Velardo. He was born three weeks before House Velardo attacked House Dellamorte. His mother, a member of House Velardo, used the chaos of the fight to flee from Treviso with her husband that day in an effort to protect her son and keep him from the life she’d been forced into. Caterina knows who Gunnar is not long after he joins the Crows as a fledgling at 15 years old, though, but she never says anything because she doesn’t expect him to survive training. When he does, it leads to her refusing to let go of House Velardo’s past actions so when she has the opportunity to give Ronan the information to use against Gunnar, she essentially gives him a contract to get rid of Gunnar, but to make his death look like it happened due to a contract gone wrong. Viago made Gunnar a member of House de Riva in an attempt to protect Gunnar from Caterina because Viago knows Caterina wouldn’t risk an outright attack against House de Riva in an attempt to get rid of Gunnar. Viago does eventually discover Caterina was the driving force behind Ronan’s actions, and he nearly starts another House war. Gunnar doesn’t find out about his mother’s past or his own ties to the Crows until right before the fight at Tearstone Island. He also doesn’t find out about Ronan’s contract with Caterina until after returning to Treviso following the fight in Minrathous. (And Lucanis is oblivious to all of this so he doesn't find out his grandmother once wanted the man he loves so dearly dead until the day Gunnar is told the truth after they return to Treviso to piece their lives back together following the fall of Elgar'nan.)