This relationship is terrible. seriously, it's so badly conceived. It just doesn't work. Therefore gwack can be used as to describe anything terrible.
It's also a great onomatopoeia for vomiting. Honestly - Gwaaaaaaaaaaaaack. Which Gwack makes people want to do.
"Seriously, I'm going to gwack"
"Gwack make me want to gwack"
It is one of the canon pairing in the series alongside Janto and Towen. (I anto Jone x Jack Harkness & Owen Harper and Toshiko Sato)
Many fans ship this pairing, but since Janto has gone canon, many have supported Janto instead.