Its was similar in some way, but it was also very different in others.

With the exception of Ghost Recon 1, which was first person, the series was always a third person shooter genre, but it occasionally was first person depending on the platform and the game (Ghost Recon Advanced Warfighter 1 is in first person only for PS2, Xbox, and PC, but is optionally third person on Xbox 360. GRAW 2 is in third person for all platforms). Most Ghost Recon games are third person, and this was likely an intentional choice to make a game that does not directly compete with Rainbow Six, another Ubisoft series.

Ghost Recon had some semblance of realism, but not on the level of Rainbow Six and definitely not on the level of Ready or Not. Rainbow Six in its later years also began to lose its realistic style and became more and more fanciful, culminating in Siege having crossovers that don't make sense for the game or genre (I love NieR, but 2B does not belong in Rainbow Six, and her model in the game looks awful anyway).

Ghost Recons biggest difference is that Ghost Recon has a military focus, whereas Rainbow Six is more focused on SWAT or counterterrorism efforts. To this end, Rainbow Six often featured levels with enclosed spaces such as the inside of buildings or airplanes and a lot of close quarters combat, while Ghost Recon favors more open maps and long range encounters. Ghost Recon also featured vehicles and vehicular combat sections while Rainbow Six generally did not. For example, Ghost Recon would sometimes have a helicopter or tank appear to assist your squad in combat, perhaps against another enemy vehicle. If Rainbow Six ever featured a vehicle, it definitely wasnt a tank assisting your squad, and at most was a helicopter shooting through building glass or something similar.