I agree. major red flag when the office manager's email address is a Gmail ones. It's 2019, if your company is still using a free email a service, than it screams either scam or fly by night operation.
Also a lack of a website is a second red flag, along with a lack of a phone number to call the company, salary information, work schedule for full time employees, or job description. I think I'll pass on this one; too many red flags.
Disclaimer: my former chief used his AOL account for everything. it was on his business card, and it worked fine, until we got a new chief, and had to updated EVERYONE with the new email address. I've finally got him to publicize the department email address; but he still forwards it to his gmail, and sends email from gmail. baby steps.....