example of how it could be done:
&(#22ff00,#0e49a6)this text starts green and ends teal

ideally would support multiple stops in the gradient:
&(&#FF0000,#ff5c00,#ff9e00,#ffe400,#9cf000,#22ff00,#158080,#0800ff,#3707d5,#6c0ea6)Rainbow text with a pretty smooth gradient

This idea is 😍

Closing in favor of #2391, as this will almost certainly not happen before then. I recommend downloading another chat plugin that already has gradient support if you want this kind of advanced feature right now on your server.