This paper presents the design of a system that supports execution of HLA distributed interactive simulations in an unreliable Grid environment. The design of the architecture is based on the OGSA concept that allows for modularity and compatibility with Grid Services already being developed. First of all, we focus on the part of the system which is responsible for migration of a HLA-connected component or components of the distributed application in the Grid environment. We present a runtime support library for easily plugging HLA simulations into the Grid Services Framework. We also present the impact of execution management (namely migration) on overall system performance.