For anyone considering it, some MS clients (and a few others) show something like :
myemail@gmail.com on behalf of Rob Aley [me@mydomain.co.uk]
It maybe looks a little unprofessional, but as long as your gmail account isn't something like offensivewords@gmail.com or cutesexyman32@gmail.com, its not too bad.
It is not too bad IMHO if you only use a slightly different domain for the same purpose but with completely different domains, it looks odd. And there is always the problem that some mail clients use your default Gmail address to reply and not the one you actually used.
myemail@gmail.com on behalf of Rob Aley [me@mydomain.co.uk]
It maybe looks a little unprofessional, but as long as your gmail account isn't something like offensivewords@gmail.com or cutesexyman32@gmail.com, its not too bad.