At the outbreak of the first world war he fled to London where he joined the D.G. Somerville & co. Contractor Company worked, first as engineer and from 1917 as Chief Engineer. After the war he returned to Ghent and was appointed in 1919 work leader and shortly afterwards to coach at the University of Ghent. In 1922 he started a free course Practice of calculation of reinforced concrete. In 1926 he founded the Magnel laboratory for concrete research to respond to the urgent need for concrete research, which in 1930 became part of Ghent University. In 1927 , he was appointed to Professor at the University of Ghent, in 1932 toAssociate Professor and full Professor in 1934 to.
In 1937 began to investigate further the Magnel prestressed concrete trial and error, which eventually led to the development of the system Blaton-Magnel for prestressed concrete.
For the University of Ghent, he did the concrete study for:
- the book Tower (designed by his colleague Henri Van de Velde)
- the University Hospital (of which his colleague Henri Van de Velde was President of the college of Architects)
- the Technicum building of the Faculty of engineering
Magnel was a member of:
- the Royal Flemish Academy of Belgium for Sciences and arts
- the Royal Society of engineers and Industrialists
- the American Concrete Institute
- the American Society of Civil Engineers
- the Institute of Structural Engineers.
He was also President of the Rotary Club of Ghent and in that capacity he founded in 1936 the Association "the friends of the Saint Nicholas Cathedral in Ghent" on, that this church has saved from destruction by to promote the stabilization and restoration of the building.
Every five years since 1959 "Gustave Magnel gold medal" awarded to the designer (s) of an executed and completed construction work that can be considered as an important and remarkable application of reinforced and/or prestressed concrete.