Greg Williamson is an unusual broker and property investor who has managed to meld together his two great loves — real estate and music. The Douglas Elliman broker’s name may be familiar to New Yorkers because, for the last 10 years , he has served as executive producer of the annual Love Rocks concerts at the Beacon Theater, along with Nicole Rechter and John Varvatos. The shows, which benefit God’s Love We Deliver, are star-studded affairs that have featured performers such as Paul Simon, James Taylor, Nile Rodgers, Cher, Alicia Keys and John Mayer. The most recent event, which took place on March 5, included performances from Mary J. Blige, the Goo Goo Dolls and Elvis Costello — with guests such as Bill Murray and John McEnroe. But now, Williamson is soon to set off on his next adventure: selling one of Brooklyn’s tallest condo towers. Greg Williamson (center) at March’s Love Rocks presentation.…