In this scene of the anime, Hiyoko has basically drugged everyone with Teruteru's aphrodisiac soup so all the characters are all hot and bothered and not acting how they normally do. Please refer to the video in the casting call's description for more details.

Gundham, who's normally well-composed, is now completely helpless against the effects of the sexy soup. In this scene, it's implied that he's struggling to stop himself from doing something... he probably shouldn't in front of his classmates. For this line, he's gripping his right arm tightly and struggling to speak as he's attempting to talk himself out of doing whatever his body had in mind.

Gundham should be voiced by a male with a deeper voice range. Exact voice impressions aren't at all mandatory, but I do want a voice that stays true to his character.

Voice reference:

https://www.youtube.com/watch?v=JpjbH-wEodY