Great apes are able to perform in imaginary scenes and role-play.
In this video, Penny play-acts with Koko, asking her to make believe Penny’s pen is a microphone. Koko plays along, and after first kissing the “microphone” does a “huff” vocalization, which she often uses when playing with a telephone or microphone. It seems to be her way of imitating talking — which she clearly understands (i.e., spoken English) but is unable to do faithfully reproduce due to the physical limitations of a gorilla’s vocal apparatus.
Note that Koko is also holding up her thumb as a pretend microphone. We know this because she uses her thumb to sign “another” on Penny’s pretend microphone, and then returns her thumb to her mouth. Also, when Penny says “right” after Koko kisses, huffs softly and sticks out her tongue, Koko shakes her head “no” and tries again, only huffing this time.