Emails not being delivered to Gmail

Users from our domain ( @gc-rehab.com) are receiving error messages that emails cannot be delivered to Gmail accounts. It appears that we need to add SPF or DKIM to our organization domain. Will you please advise how to make this change? 

The error message says: 

Diagnostic-Code: smtp; 550-5.7.26 This mail is unauthenticated, which poses a security risk to the
 550-5.7.26 sender and Gmail users, and has been blocked. The sender must
 550-5.7.26 authenticate with at least one of SPF or DKIM. For this message,
 550-5.7.26 DKIM checks did not pass and SPF check for [gc-rehab.com] did not
 550-5.7.26 pass with ip: [66.96.187.6]. The sender should visit
 550-5.7.26  https://support.google.com/mail/answer/81126#authentication for
 550 5.7.26 instructions on setting up authentication. i22-20020a170906851600b00987d66e6d26si4346920ejx.250 - gsmtp