Gijs studied experimental physics in Leiden, where he obtained a master's degree in 1981. His master's thesis was on a subject in low-temperature physics. In 1986, he obtained his PhD degree from the University of Amsterdam on a topic in particle physics for which he did the experimental work on the NA32 CERN experiment via the Dutch NIKHEF institute. Between 1986 and 1988, he worked as a CERN fellow in the SPS division on the design of the first LHC dipole model magnets. In 1988 he switched to accelerator operation and worked in the SPS-LEP control room on the commissioning of the LEP collider, the development of the SPS accelerator and the beam transfer lines. From 1996 until 2002, he was responsible for designing and constructing the resistive magnets for the LHC ring. In 2003, he returned to superconducting magnets and was the project engineer for one of the three industrial supply contracts for manufacturing the LHC dipole magnets. Between 2007 and 2014, he led the development of high-field accelerator magnets at CERN, for which he also led the high-field magnet activities in the EuCard project. In parallel, he was, from 2011 until the end of 2017, the deputy group leader of the CERN magnet group and, from 2013 until the end of 2020, section leader of the magnet development section in the magnet group. From 2017 until his retirement in April 2023, he ran a development program for high-field HTS magnets at CERN. 

Apart from his research, since 2012, Gijs has been a dedicated teacher, sharing his expertise and knowledge with numerous students. His teaching experience spans over a decade, and he has been instrumental in inspiring and mentoring the next generation of physicists.