Global Software Development (GSD) is becoming increasingly prevalent, with software development teams being distributed around the world and working in collaboration with partner companies despite geographic and time differences. The main advantage of GSD which makes it attractive is the greater availability of human resources at lower costs. However, there are several disadvantages which are caused by the distance which separates the development teams. Coordination and communication become more difficult when the software development teams are located in different places, resulting in hidden costs involved in this process. As such, the effort estimation models used for collocated software development are inadequate for estimation in GSD. Thus, effort estimation in GSD is becoming an important area of research. Many researchers have focused on effort estimation in GSD over the last decade. This paper presents the findings of a systematic review of the related literature by summarizing the hidden costs in GSD, and discussing the open research issues in effort estimation in GSD.