Final answer:
The bank may have refused to pay Doug the $500 check because the check was not properly endorsed by Gail.
Step-by-step explanation:
The reason why Gail's bank refused to pay Doug the $500 check could be because the check was not properly endorsed by Gail. Endorsing a check means signing the back of the check, indicating that the check is being transferred to someone else or being deposited into their account. Without Gail's endorsement, the bank would not release the funds to Doug.