I re-installed Google Drive for my mac (OSX Yosemite 10.10.5) and the icon has since disappeared from the favorites bar in the finder. However, the icon shows up in the more info section. What is going on here? Here is the view of the Favorites bar in Finder.
And here is the more info view that shows the icon.
This's actually by design since macOS has stopped showing custom icons in recent versions of the OS.
My Google Drive icon is also a generic icon, others too:
If you want to display the Google Drive icon, it wouldn't be possible without third-party software.
Extra: Dropbox uses a "hack" to show its icon in the sidebar as seen in this question, How come Dropbox gets an icon in Finder sidebar?.
