Massive stars are fundamental engines in cosmic evolution, providing strong radiative and kinetic feedback to their environment through their copious ionizing radiation and energetic explosions. They also drive the chemical evolution of their hosts. Despite their importance, large uncertainties remain in our understanding of massive stars, including processes such as mass loss, interactions with close companions, and the formation of compact objects at the end of their evolution. The detection of gravitational waves from merging compact objects introduces an important new avenue to study these processes, with an associated large effort from the astrophysical community to understand both how these sources are formed, and how the observed sample can be used to constrain the existing uncertainties. Key questions that are critical to resolve these problems are:

- Which intermediate phases in the formation of gravitational wave sources can anchor their evolution?
- Can we identify individual progenitors of gravitational wave sources in the nearby universe?
- How do the final properties of a massive star relate to the resulting compact objects they form?

To address these, the STAR-GRASP project aims to produce a novel theoretical framework to connect constraints from electromagnetic observations to the observed sample of gravitational wave sources. To achieve this we will perform extensive simulations of single and binary star evolution, covering their whole life from birth to their final death-throes as merging compact objects. This will provide multiple predictions on the properties of binary systems with at least one compact object, as well as on the electromagnetic transients associated to their formation. Our new theoretical predictions will be testable in the coming decade, with the advent of large-scale multi-epoch surveys of stellar systems and transient electromagnetic events, as well as the rapidly growing sample of observed compact object coalescences.