Translation:One kilogram of beef, please.
ぎゅう is the on'yomi (Chinese reading) of 牛 which means cow. The kun'yomi (Japanese reading) is うし. Normally in Japanese kanji that are alone are read by their kun reading, and kanji that are part of a multi kanji phrase are read by their on reading. So if you are talking about a cow you'd say うし, if you are talking about cow meat or milk you'd say ぎゅう(and then にく or にゅう)