According to Urban Dictionary, "gagged" means "to be amazed or stunned by someone or a situation of some kind." The site says ...

As with all slang, if you need an online list to know what a word means, you are too old to say it aloud. 304: Hoe. (Type "304" on a calculator and turn it upside down.) 4+4: Ate. Four plus four ...

The term “slay”, refers to someone who is stylish or successful. It can also mean to impress someone very much or to be very good. It was voted children’s most preferred slang term.

Despite the lack of a clear-cut meaning, it was selected as the favourite slang term of 15 per cent of the respondents.