I don't have gMail on here. Unless someone with it can help you. You can usually get to it by going to your Applications, right clicking on the App (gMail) and clicking 'Show package contents'. From here there's usually a folder called 'Resources', look for that and within it you should find a .icns file.
Again no idea if that'd work but I don't see why not, barring filesize/pixel restraints that could be on there. If you want to try though just open that .icns in PS or something, make an edit and resave it (Obviously make a backup). Could work right?
Oh I assumed you'd downloaded the App. In which case I'm out sadly! I have no idea if you can set a weblink to a dock icon, but there's probably some way of doing it (I now understand why you wanted to know about an app ). Sorry about that! Hope you get some help soon.