Gemini Man is the result of "sunk cost fallacy". The movie concept was originally pitched to Disney back in 1997. When you're sitting on a movie for that long, it's generally a bad sign. Especially when it's essentially just your standard action movie.
Gemini Man does very little interesting outside of the technology behind the movie. The CGI "young Will Smith" looks pretty good - except during bright scenes. The movie makes up for this by never having light scenes with young Will Smith until the end. (And boy does it look bad at the end). Gemini Man was also filmed at a higher framerate (120FPS), but most movie theaters didn't have the technology to display 120FPS, so they just displayed it at a more normal FPS.
Outside of the technology, this movie is very run of the mill for an action movie. The most memorable things are things that were "so bad it's good". (A ridiculous fight scene about 30 minutes into the movie, and some over the top product placement). But for the large majority of this movie, I was just bored.