The COST (European Cooperation in Science and Technology) homepage describes this cooperation as 

"Growing ideas through networks

The European Cooperation in Science and Technology (COST) provides funding for the creation of research networks, called COST Actions. These networks offer an open space for collaboration among scientists across Europe (and beyond) and thereby give impetus to research advancements and innovation.
COST is bottom up, this means that researchers can create a network – based on their own research interests and ideas – by submitting a proposal to the COST Open Call. The proposal can be in any science field. COST Actions are highly interdisciplinary and open. It is possible to join ongoing Actions, which therefore keep expanding over the funding period of four years. They are multi-stakeholder, often involving the private sector, policymakers as well as civil society.
Since 1971, COST receives EU funding under the various research and innovation framework programmes, such as Horizon 2020.
COST funding intends to complement national research funds, as they are exclusively dedicated to cover collaboration activities, such as workshops, conferences, working group meetings, training schools, short-term scientific missions, and dissemination and communication activities. For more information, please go to the Funding section of this website.
The COST Association places emphasis on actively involving researchers from less research-intensive COST Countries (Inclusiveness Target Countries, ITC). Researchers from Near Neighbour Countries and International Partner Countries can also take part in COST Actions, based on mutual benefit. For more information, please visit the global networking page."

The RTG MoG is involved in the Action 
CA16104 - Gravitational waves, black holes and fundamental physics  

"Gravity claims a central role in physics. Almost every grand challenge in astrophysics, cosmology and fundamental physics includes gravity as a key ingredient, making it a subject of strong interdisciplinarity. Black Holes (BHs) are the atoms of General Relativity (GR). They are the “simplest” astrophysical objects, yet they harbour the most remarkable predictions of GR: event horizons and singularities. Gravitational-wave (GW) astronomy – made possible by the historical detection of GWs in 2016 - will allow us to test models of BH formation, growth and evolution, as well as models of GW generation and propagation. It will provide evidence for event horizons and ergoregions, test GR itself and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to answer some of the most pressing issues in our understanding of the cosmos and the laws of Nature. The scientific prospects and exciting opportunities have potential paradigm-shifting consequences in astrophysics, cosmology, and fundamental physics."

More information can be found e.g. here.