They are two very different things even though they share the keywords "graph" in them.
CosmosDB Graph API is a query language to manage and query "CosmosDB graph database". Graph database unlike traditional databases, store information in the form of a graph (i.e., vertices and edges).
GraphGL on the other hand is a query language for a user API. Let's say that you have a build stack (the underlying platform could be anything) that has a REST endpoint. However the rest APIs typically are simple and doesn't let you specify complex filtering conditions. GraphQL is a way (or replacement to tradition REST API) by which the developer of the stack can allow clients to avail these complex functionalities.