After Joshua turned down Rachel's marriage proposal, Gunther shouted, “You idiot!”
The moment occurs in “The One with All the Wedding Dresses,” a fourth-season episode of Friends. Rachel Green becomes swept up in the idea of marriage after seeing Ross and Emily's engagement plans. While dating Joshua, she proposes that they get married, but Joshua is startled by the sudden suggestion and rejects it.
Gunther, the Central Perk manager who secretly loves Rachel, reacts with immediate disbelief. From his point of view, Joshua has been offered exactly what Gunther would want: a chance to marry Rachel. He therefore runs after Joshua and calls him an idiot, turning Gunther's usually quiet jealousy into an unusually direct outburst.
The joke works because Gunther is not simply defending Rachel's feelings. His reaction reveals how strongly he idealizes her and how absurdly obvious the opportunity seems to him. Viewers sometimes remember the line as being directed at Rachel or Ross, but Gunther is speaking to Joshua for refusing Rachel's proposal.