Answer:
Dean and Donna can sue GSI because they breached their contract since GSI did not provide the information for which they were contracted for.
The report given by GSI probably made Dean and Donna overpay for the house. Since Dean and Donna suffered a financial loss, they can sue GSI for compensatory damages. The compensatory damages would be the difference in value of the house if it had the septic system described by GSI and the house without that system.