Technology has come a long way in the movies. Practical effects, where they set up real models, stunts, sets, etc. and film them getting blown up or whatever, are getting better, but so are computer enhancements. But people are still too tough for them to make well. Remaking a youthful Leia in Star Wars, removing a moustache from Superman, and now creating a young Will Smith... each of them are really impressive, but still not quite right. And these failed effects will pull you out of a movie. As will poorly executed plot lines. And this movie is full of both.
Gemini Man follows the retirement of an assassin who finds out he killed someone that wasn't quite as bad as he was lead to believe. Those in charge try to cover their crime by killing anyone associated with him, but he inevitably escapes. So they have to send in their BEST! His clone! Can youth overcome experience?
This movie has a lot of problems with plot and character development. The action isn't lacking, though, so that can be entertaining. But there are just a lot of silly mistakes that just don't make any sense. A super gun firing a zillion rounds tears through a building. But when in the building, the bullets are coming from all over even when only one gun is firing. Why are they shooting from every different angle? There's tons of stuff like this in the movie. Like they just didn't care enough to consider reality. They had their story and a few ideas about how it would develop and just went with it. Maybe they spent their budget on making the digital Will Smith and couldn't afford a good editor. Regardless, the action is there, so if that's all you care about... well, you could probably watch several dozen not-so-great action movies on Netflix and save your money for a better action flick.