In order to establish a graded reasoning mechanism and provide a possible framework for approximate reasoning in n-valued propositional logic, this paper introduces the concept of α-truth degrees of propositions in n-valued Gdel logical system by using the infinite product of uniformly distributed probability spaces of cardinal n. It is proved that the general inference rules with truth degrees hold, and a sufficient and necessary condition to judge α-tautology is obtained. Moreover, an intrinsic pseudo-metric on the set of propositions is defined by means of the similarity degree between propositions, which makes it possible to develop approximate reasoning in n-valued propositional logic. The graded method proposed in this paper lays a foundation for the algorithmic realization of approximate reasoning and serves as a guideline for the graded reasoning about knowledge.