A GSP file is an error message file that indicates a problem with the delivery of the email. It is sent by IMail Server, an email server program that is used for email communication by numerous small and medium businesses. The error could be due to a number of reasons such as huge amount of data, loss of data in the MSG file, or out of memory errors.
GSP File Format - More Information
GSP files are stored in web file format and are easily viewable with email clients such as Microsoft Outlook. A GSP file is generated as response of a sent email that results in an error.