OSI GS-121 Gutter & Seam Sealant OSI GS-121 Gutter & Seam Sealant is composed of SBR rubber and high quality resins to form a tough, rubbery seal that expands and contracts with the change of weather without cracking. It is formulated especially sealing aluminum, galvanized and vinyl clad gutters and down spouts. Also available in VOC compliant formula.