Most error messages shown from PayPal will give a generic result number and a more specific Error Code. For example:

Result Message: 12
Error Code: 15005

Look up PayPal error codes at the following site: https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_errorcodes

From there, determine if it is a user issue: declined, invalid card, need zip code, etc. If it is a user error, report the message to PayPal.

If the error is some type of account error such as "invalid paypal ID" or "login information incorrect", then contact GSL to remedy the issue.