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 communities such as diaperswappers.com, sewusa.com and fixya.com are good places to look for Serger sewing machine manuals and are also fun places to hang out and swap advice. Just join up and post a question!
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: