Hello, I would like to know if the website Pokegts is harmful or could..

It is a site used for hacking the game. It is very dangerous. Your save file is in danger, but your DS might not be.

Even if it does not harm your save file, your Pokemon (or even possibly your whole game) will become ineligable for tournaments.

You (and others reading this) may claim that they've never damaged their save file through hacking, but they are the lucky few.

If you decide to take the risk and do damage your save file, you may not be able to fix it.

If you have already used it, I would suggest releasing all hacked Pokemon and getting rid of all hacked items. That usually is enough to prevent new problems from arising. To be as safe as possible, you would need to delete your save file and start a new one. Keep in mind, hacked Pokemon/items can still damage games even if they originated on a different game.