Simone Favaro, Kieran Low, Grayson Hart and Mike Blair have already signed deals to join the club next season and Gregor is pleased with his latest additions.
Speaking exclusively to glasgowwarriors.org, he said: "Both Sam (Johnson) and Greg (Peterson) will be great additions to our squad next season and we're looking forward to them joining us.
"Sam has played very well since his return from rugby league, producing some eye-catching games in the NRC and at club level.
"He has been a regular for the Reds back-up side this season and his impressive performances have seen him break into the first team.
"He is a strong running ball carrier that can cover both centre positions and we believe that he will continue to improve within our environment."
USA international Peterson has spend the last four months at Leicester Tigers and Gregor believes the 6ft 8in lock will provide more depth in the second-row.
He added: "Greg has picked up some great experience already in his rugby career and played well for the USA in the November Tests. He is determined to make the most of his opportunity with us.
"He has a real physical presence and hits hard in the tackle. He will give us something different and add to the competition for places in the second-row.”