Can i log into my account with only using my username/gamertag?

You still need the password

What about you know the username and the gmail ?

No. You need the email and password to be able to login.

When I look in the Microsoft store I can see that I'm logged in with the same email address, but there's no username or gamertag of any sort displayed, it only displays my real name. I do have my name tied to the Xbox account with my gamertag, however, and it as well is what shows up when I'm looking at my purchase history, so I'm assuming I am indeed logged in with the same account.

As a side note, your Gamertag is the username for the forums here. So you just need to log into your regular account using the credentials for your Microsoft Account that your Gamertag is associated to. No one else can take your Gamertag unless they have
your username and password. The account is already created for the forums when you made your Gamertag, you just need to log in with it.