Gemini Man aspired to be more than just an action flick, however with lack of story and character development, the third act and movie falls flat.
The trailer says it all, old Will Smith against young Will Smith. The idea presented is that our own worst enemy is ourselves. A great concept, but poor writing and plot holes causes the movie to feel very clumsy. And worst of all, the special effects look worse than most TV produced movies. The young Will looked odd at times and the final battle reminded me of the CGI used in Blade 2, yes that bad.
If you are looking for an action flick to watch, I say you can find better choices on Netflix. You might even give 6 Underground a try over Gemini Man.