You do not want to set up your work email to forward messages to Gmail
automatically.
Rather, you should set up y our work email to *re-direct* messages to Gmail.
Forwarding attaches new SMTP headers to the email message, hence the
From: field will be your work address, the subject field will have a
FW: on it, etc. Think of it like receiving a letter, then putting the
letter in a new envelope with the forwarding address (your Gmail
account) and a *new* return address (the From:, i.e. your work
address) on it, and sending it on. Since it's got a new return
address, replies go back to your work address instead of the original
"return address."
Re-directing, on the other hand, passes the message along without
making any changes to the relevant SMTP headers (From:, Reply-to:,
Subject:, et al). This is like receiving a letter, writing the
forwarding address on the outside of the original envelope, and
sending it on. The return address hasn't changed, so replies go to the
correct place -- the original return address.
Unfortunately, some email systems don't make a distinction between
forwarding and re-directing in their interface (even though the SMTP
protocol does make distinction).
--g