To reach the GM supervisor team you would fill out the survey you get when your ticket is closed.
Please keep in mind that GMs do not have the ability to fix bugs, nor do they have the ability to award things that were not actually granted by the game. It does not matter if the reason for the lack of reward is that a bug caused it.
The most they can do is tell you to please report it as a Bug on the Bug Report forums, or in-game. The Devs are the ones who fix bugs, but usually that is not retroactive and won’t grant you rewards that were not earned due to a bug. You would have to repeat the event.
If you keep putting in tickets for the same thing, after being told what they GMs can, or can’t do, then it does cross the line to GM harassment. Sometimes the answer is that they can’t help you. They would love to give you what you want, but they can’t either due to lack of logs/data or due to policy.
If you did not complete an event, kill a boss, earn a reward, get an achievement - they can’t grant it to you.
You can read more here about what GMs can, and can’t do. Their powers are actually quite limited, esp for in-game things.
Edit to add - the only way you will get a resolution is if the Development team fixes the bug and the fix includes granting the rewards. That is in their hands, not the hands of GMs.