GoKawiilThere have been multiple leaks in the lead-up to today's official reveal of more gameplay. While damaging and frustrating for the dev team, those were at least benign for players. Rockstar has not released any demos, so sites claiming to have one are likely trying to scam you. Similarly, the person or people responsible for the recent leaks only published videos and screenshots — there is no build or demo available online.
No matter how much you want a first look at Lucia and Jason in action, keep your wits about you. Double-check that a site is legit before downloading files or entering passwords. And if the file seems way too small for what will surely be a storage-hog of a AAA video game, don't download it. If you want to know all about today's GTA 6 "extended look," have a read through our similarly extended breakdown.