What lurks behind these Gmail/GTalk permissions?
More than one year later, and no answer here. I've done some research myself – and though there was not much found, I'll share what I've got so far:


com.google.android.gm.permission.READ_GMAIL: A good guess is this targets the mail content. Protection level switched from "dangerous" to "signature" about the time Gingerbread was released (with Gmail 2.3.5), see Android Census – so it's not available to third-party apps (anymore) – and thus not relevant unless we're talking about system apps.
com.google.android.gm.permission.WRITE_GMAIL: Permits an app to change your E-Mails in Google Mail. This includes sending and deleting.Assigned protection level differs between devices (dangerous/signature), so no general say on whether it affects non-system apps.
com.google.android.gm.permission.READ_CONTENT_PROVIDER: This is primarily for access to information on Mails in Gmail. Developers can use this content provider to display label information to the user.Protection level is "dangerous" – so unless that's changed meanwhile, this one definitely affects user-apps.
google.android.gtalkservice.permission.GTALK_SERVICE: I couldn´t find any documentation on this permission, which is connected to Google Talk resp. Google Hangouts.The protection level assigned seems to differ between devices – so again no clear rule here.