The 25-year-old number eight is in his second season with Saints, and he will move to Sixways this summer.
"It is fantastic that GJ has decided to join us," Worcester rugby director Dean Ryan said.
" He is an experienced Premiership player with strong leadership qualities who we see being influential on the pitch as well as joining a strong leadership group to help us build a new culture here at Worcester."
Worcester have also announced that Rotherham scrum-half Charlie Mulchrone, Ospreys prop Joe Rees and Plymouth Albion centre Heath Stevens will join them at the end of this season.
- Sports & Recreation
- Worcester rugby
- Dean Ryan