Is it possible to launch an incognito chrome window which is logged specifically into a certain google account (loading, say, gmail.com)?
No. In incognito mode history, cookies and cache are deleted as soon as you close browser. Since history and cookies are deleted you can't have incognito mode start with specific logged in Google account.
But in normal mode you can have this type of setting. Link on how to do it in normal mode
Make sure you have logged in with Google and don't delete cookies
No. Is not possible. Incognito mode generates a new and temporal blank session (cookies, local storage, etc.) which is precisely the objective of incognito mode, remove any identifiable information and give yourself a clean slate start.