Patrick Gallagher, head of Milwaukee real estate firm Siegel-Gallagher, will leave the company to join the Milwaukee office of CBRE. Gallagher, president and CEO of Siegel-Gallagher, has been focused on selling investment properties, which he said is increasingly becoming a global trade with big data requirements. Compared with a relatively smaller local operation of Siegel-Gallagher, the global CBRE firm is better suited for that kind of brokerage, he said. “I have clients in five different…