I have a Gmail account and out of the blue when replying to a client, it added an email signature that I never created, not to mention it was filled with typos. It only happens with this one particular client. I have no idea where it came from nor how to delete it. Any thoughts?
This case sounds like a classic case of a malware infection. You should try running an anti-malware tool such as Malwarebytes. The free version will perform the same scan as the paid versions and will most likely be able to clear up your problems. This tool is the pretty standard tool for removing Malware however others do exist.
(I have no affiliation with mentioned software.)