Gmail does show the originating IP. Test it yourself. Send a message to somebody, anybody. Then go to the sent email, click the dropdown arrow, and choose "Show Original". 
The raw source of the email will show. This is similar to the way the HTML source of a webpage works. The IP address you sent from will be the "X-Originating-IP" value in the email header. Other identifiable information could be injected into email headers as well, such as browser information, but I don't think Gmail does this. In the one I just tested sent from Gmail, no identifiable information is there other than your IP.
To be safe, I would never respond to Gmails unless I am behind my stealth IP. I personally connect to eBay through a VPS, which gives me a stealth IP - if I respond to Gmail messages behind the IP, it would just look like a normal email response.
EDIT: Weird, I just tested this through my VPS and my IP address did not show up in the email headers - there is no "X-Originating-IP" value at all. However, when sending from my home IP, the IP did show up in the email headers. Both are Gmail accounts. Any idea why the difference?
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