Only the Google Voice account that shares your Gmail username will be linked for making or receiving phone calls in Gmail. For example, if your Gmail address is example@gmail.com and this is the same username you utilize to sign into Google Voice, then you'll already be linked.
However, if your Gmail address is example@gmail.com and your Google Voice account username is example@domain.com, than you will be unable to link these accounts. At this time, it's not possible to transfer Google Voice accounts between Google Accounts.