The task of a MAC protocol is to manage the access to the (wireless) medium so that it can be shared by multiple nodes. TDMA MACs achieve this by providing non-overlapping access schedules. Typically, such schedules are dictated by a set of master nodes or negotiated between the nodes in a preliminary phase by exchanging messages. This has to be periodically repeated to adapt to changing network conditions. In this paper, we present a MAC protocol called GMAC, which assumes that nodes know the geographic positions of their two-hop neighborhood and exploit this knowledge to construct a TDMA schedule. The schedule can be computed locally, no negotiation phase (and thus, no additional traffic) is needed. Additionally, GMAC guarantees a maximum delay per node which makes it suitable for data with real-time constraints. Further, GMAC allows nodes to dynamically utilize time slots assigned to but not used by neighbor nodes. Finally, it permits nodes to sleep when they are not involved in communication, thus greatly reducing energy consumption.