[1] In October 1948 Theodore von Kármán was invited by the Instituto Nacional de Técnica Aeronáutica (INTA) to give a series of conferences, during which he met Millán and they starting a close professional and personal relationship.
At the Sorbonne University in Paris during the academic year 1951–1952, von Kármán gave a seminar, in which Millán collaborated.
In 1961 he was elected a member of the Real Academia de Ciencias Exactas, Físicas y Naturales (Royal Academy of Exact, Physical and Natural Sciences).
[1] In 2007 the Universidad Carlos III de Madrid created in his honor the "Gregorio Millán Barbany" Institute for Modeling and Simulation in Fluid Dynamics, Nanoscience and Industrial Mathematics.
They had a daughter Marta and a son Teodoro, named after Gregorio Millán's teacher Theodore von Kármán.