Sword Art Online vs Gun Gale Online: What's the difference between the two games?

In the world of Sword Art Online, VR technology and by extension VRMMORPGs are the very foundation of the narrative. While the technology is extensively showcased in various ways, VR Games seem to be the most prominent means and within that subset, two games are the most important ones for the plot - Sword Art Online (SAO) and Gun Gale Online (GGO). This raises one crucial question, Sword Art Online vs Gun Gale Online: What's the difference between the two games?

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