The partnership would create "the resources and capabilities to provide services at scale and safely through one team", said RES.
RES Offshore, part of the developer RES, offers developer, engineering and maintenance services to the offshore industry.
Jim Sandon, current COO and next CEO of RES Offshore, said: "Our offshore and engineering expertise unlocks the market for GES to deploy their considerable experience."
GES maintains more than 12GW of wind energy assets, according to RES. The firm has also installed approximately 22GW of wind capacity worldwide.
Most of this, however, is located onshore. The GES website shows is has worked on one half of the 60MW Alpha Ventus offshore project in Germany. The firm helped to install and service the six Areva (now Adwen) 5MW offshore turbines.
RES said the move was as a result of more projects reaching the end of the warrenty and new project owners and financial structures give "opportunities for large scale independent service providers".
"Creating the alliance allows each company to access a market more quickly than if each were acting alone," a RES spokeswoman said.