There is a bug with Pidgin, where the certificate for gmail.com cannot be validated.The problem is discussed here and there is also a solution:openssl s_client -showcerts -servername gmail.com -connect gmail.com:443 That outputs the certificate in terminal as it should, like here, but I don't know how to add that to Pidgin for a gmail account.At the ticket/bug link it says this will retrieve correct certificate with CN=gmail.comafter importing it into pidgin (Tools->Certificates) it should work  fineI have read that it should be saved in a text file in ~/.purple/certificates/x509/tls_peers that should be called username@gmail.com, but that doesn't work.
Import gmail certificate in Pidgin
ssl;pidgin;gmail