Occam's razor attempt at explaining this. You are connected on LinkedIn, your client got your gmail from your LinkedIn profile, and now the gmail address is set

Occam's razor attempt at explaining this. You are connected on LinkedIn, your client got your gmail from your LinkedIn profile, and now the gmail address is set in the "primary" field of their contact card for you. My 2 cents, without having any more context than what you described. There are more elaborate/automated versions of what I described in B2B marketing (like auto update of CRMs based on LinkedIn data). Pretty standard stuff. I tend to stay connected with business acquaintances when they are in between jobs, and in those periods, i keep their publicly given Gmail on file to engage with them. I try to update with new pro addresses when they find a new gig, but the update is not always systematic.