Complex sentence: Unlike Gamescom and other video-game trade fairs open to the public, E3 is an industry-only event; individuals who wish to attend are required by the ESA to verify a professional connection to the video-game industry.
Split into simpler sentences: Unlike Gamescom and other video game trade fairs that are open to the public, E3 is an exclusive, industry-only event. Persons who apply to attend are required by the ESA to verify that they have a professional connection to the video game industry.