Many gifted students "Dumb down" to fit in, especially in early ages when fitting in is more important to them. Feeling they need to do so itself can be frustrating.
Extra work or worksheets that simply reinforce math facts or crossword searches on the topic one has already mastered is also frustrating. We most often refer to them in our household as "Busy work" teachers give advanced students to keep them busy while they teach down the center line. We once had at the start of the next school year a teacher who want to start the "Math facts" system (Addition, subtraction, multiplication, division timed tests) all over again even if the students had already mastered them "Just to make sure." (and we "Made sure" that didn't happen)
Reinforce with your child that it's okay to be different than others, in fact, that's what makes life interesting. Explain that everyone has their strengths and challenges, they're just different for everyone. It's okay to stand out and there are lots of very smart people in the world and there is room and appreciation for all of them.
You might also look for positive ways to let frustration out. Plenty of exercise certainly made a difference for our eldest DS in that manner.
