Mojang's migration FAQ says that you get to keep your in-game username (I presume that's what you mean by "gamertag") but there's no guarantee that you'll be able to get the same name for your Xbox Gamertag, if you're worried about that you should register a Microsoft account now to increase your chances of either getting the same name or something else that you can live with.
quite the opposite, i wanted to NOT have my xbox gamertag, its literraly technoblade6644 and i cant change it cuz my brother used all the gamertag changes lol, i have to pay money if i want to change my gamertag again.. And that sucks
The Meaning of Life, the Universe, and Everything.
No. Definitely not. Gamertags are a dime a dozen, and that would send the Minecraft community into an uproar, if your Minecraft name became your gamertag. Mojang/Microsoft actually listens to its community, and they have already addressed that here.
A direct quote from the official FAQ page linked above:
DO I GET TO KEEP MY MINECRAFT: JAVA EDITION USERNAME?
Yes, you will be able to keep your in-game username for Java Edition.
During the account migration process, you’ll also need to pick an Xbox Gamertag. This might differ from your Minecraft: Java Edition username, if that one was already taken or if it doesn’t meet the Gamertag character requirements.
So, you will keep your current Minecraft account name, it's just that your gamertag will be different. Gamertag does NOT equal Minecraft username.