Gmail application supports IMAP server where you can configure other account into the Gmail or connect your Gmail account with other IMAP supported platforms. You can also connect two Gmail account together with the same way, but doing manually may cause issues. This is why the professsional method is the best way. Corbett Gmail Migration tool can hlp you with that. For more information read the blog below.
https://corbettsoftware.com/blog/merge-two-gmail-accounts/