The 36-year-old Norwegian joins the British squad after one season at FDJ-BigMat.
Rasch is widely regarded as a Classics specialist but has made five Grand Tour appearances.
Rasch, who was national champion in 2003, is a former team-mate of Edvald Boasson Hagen and Lars-Petter Nordhaug at Team Maxbo Bianchi.
His former teams also include Credit Agricole and Cervelo which would later morph into Garmin-Cervelo.
Team Sky coach Rod Ellingworth said: “It’s great to have Gabriel on board. Not only will he be a huge asset to us in the Classics, his attitude and experience will be of massive benefit to the team.
“The riders and performance staff that have worked with him in the past only have good words to say about him, and having watched him closely over the last 12 months, we know he has a great deal to offer the team.”