Additionally, Maria Robotnik was not killed by G.U.N. directly. It was Professor Gerald Robotnik, her grandfather, who, in an attempt to escape with her from the space colony, "The ARK", accidentally triggered the "Final Chase" protocol. This protocol initiated a self-destruct sequence, resulting in the destruction of the colony and the death of both Maria and Gerald.
However, G.U.N. was present on the ARK at the time and played a role in the events leading up to Maria's death. They were initially tasked with capturing Gerald and Maria but ultimately chose to follow the orders of the then-president of the United States, who ordered the colony destroyed to prevent the use of the "Chaos Emeralds" for weapons.
In a nutshell, while G.U.N. wasn't the direct cause of Maria's death, their actions and decisions contributed to the tragic events on the ARK.