When you are missing a manual of any sort, the online community can help you out... Several sites can assist with your Gemsy RXM5DA Serger, such as mastersewusa.com, who
have a large store of sewing machine manuals, although they do charge a fee if you want a hard copy sent to you via the post.
Online auction sites such as eBay often have Gemsy products for sale, and if you find someone selling a Gemsy RXM5DA serger, you could ask if they have the manual and would be willing to send you a copy.
Gemsy don't appear to have a customer service center in the USA, but if you still can't locate a manual, there's always the YouTube community. Videos such as this one are watched by Serger users, so post a comment and you may be in luck: