When Galladon could see Loras' face distorted by anger he was sure his tactic worked. Obviously, he didn't want to disrespect the Lord of the Stormlands for no reason. Because House Tarth was extremely low profile in the last decade many people might have forgotten about them, apart from the merchants that were in regular contact with Tarth.
As there were no orphans or beggars in Tarth and commercial trade was limited to the Sapphire Hall, Galladon was sure that even Varys and his little spies had absolutely no knowledge about what happened on the island of Tarth. The only source of information would be the merchants and slave traders. But Galladon didn't want to walk into King's Landing having to deal with any offenses done by some mindless nobles.
It would become very bothersome if he had to deal with some nobles that offended him, mostly some Lannister people. Therefore, he wanted to make his presence and strength known before he reached his destination. Having a small clinch with Renly Baratheon could be explained by two young proud man facing each other for the first time. He fully expected Loras to challenge him to protect the honor of his lord and lover. Fortunately, the young and impulsive son of House Tyrell didn't disappoint him in the slightest.
"Galladon Tarth! What do you mean by carrying swords into the dining room of Strom's End? Do you want to disrespect Lord Renly, or do you threaten his safety?" Loras said, fuming full of anger. While talking he stepped in front of Renly, protecting him behind him, already drawing out his sword a little.
When the guards outside the hall heard the sword drawing out of his sheath, they left their position and entered the hall. The situation was changing very quickly and even Renly and Loras themselves were quite surprised when suddenly soldiers surrounded the young heir of House Tarth and his guard in the middle. Obviously, the soldiers of House Baratheon haven't dealt with a similar situation in many years, somehow acting a little too quickly and aggressively in this scenario.
The dining room suddenly became eerily quiet. Nobody wanted to be responsible for a sudden escalation of the situation. Loras Tyrell felt extremely uncomfortable, like riding a tiger. He was indeed young and impulsive man but trained in politics and noble conduct by Lady Olenna since his early teens. Certainly, he didn't want to be responsible for a real dispute between House Baratheon and House Tarth.
When the situation was stale Renly Baratheon slowly stood up from his seat trying to maintain a dignified expression, but a sudden sound nearly caused his heart to jump out of his chest. While everybody was focused on the center of the dining room a guard from outside entered the room from a different door.
The scene in front of him looked like a real confrontation with both parties being armed. As a response the guard used his bow and pulled the bowstring, ready to intervene at any point in time. But this sound of pulling the string was like a signal, turning Gustav from a leisure person into a demon.
Without waiting for any order or giving anybody time to react, even Galladon, he pulled a short spear from the quiver on his back and threw it straight at the right shoulder of the bowman. This poor little guy had no idea what was happening to him. Before he could even fathom the movement of Gustav his right shoulder was pinned to the wall by the throwing spear. He cried out in pain and the arrow of the bow shot out in Galladon's reaction. With his inhuman senses and reaction speed he could have easily avoided the arrow altogether, but he wanted to use this opportunity to escalate the situation a little further. He just stood there waiting for the arrow to hit fully trusting Gustav to take care of the situation. Obviously, his loyal guard would never disappoint him and easily crushed the arrow with the tip of his long spear before it could harm his young lord.
Truthfully, even if the arrow hit, if Galladon avoided direct impact with his more sensitive eyes, his evolved skin alone would have been enough for him to defend himself from any serious damage. But he didn't want to show his speed or defensive capability this early, so choosing to wait for Gustav's protection was the right call in this situation.
The shrill screams of the bowman was in large contrast with the silence of Renly's guard, Loras Tyrell and Renly themselves. The guards who came in first were completely stunned, not knowing whether to attack the heir of House Tarth and his guard, while both Renly and Loras were flabbergasted by the situation. The young Renly simply didn't know how to proceed, while Loras was really frightened.
If Gustav just threw the spear straight at Renly's chest, Loras would have no way to even try to defend this. From the speed of the attack and the heavy impact the throwing spear had, when facing the strong castle wall, it might have even just cut straight threw his iron armor.
While a few second went by, feeling like ages, Galladon chose to remind the young lord of the Stormlands:
"If you don't call for a medical practitioner very soon the life of your bowman is lost in a few minutes. If you hurry, he might have the slightest chance to survive."
Suddenly, everybody focused back on the crying howls of the bowman, who was nailed to the wall and desperately struggling. After taking a few moments to process the current situation Renly turned back to his former self, telling the guards to get the Maester of Storm's End, who was very familiar with healing wounds.
Renly didn't bother to take care of Loras' hints not to send away his guards because he was very sure Galladon Tarth would not harm him in the deepest place of his castle. It would simply be pointless to offend the Lord of the Stormlands and the youngest brother of Robert Baratheon in this way. While the guards left only Renly and Galladon, with both of their personal guards, were left in the room.