Hyperledger Fabric does not offer a REST API natively. As noted, it uses gRPC for all interactions with the outside world.
However, Hyperledger Composer does have a means of generating and exposing a REST API for your application using Loopback/OpenAPI (Swagger) that you can then call from your client application (or expose as a web client). Further, there is work being done on developing a REST service sdk for Fabric.
While neither of these is mature, yet, the Composer option is more mature than the sdk, and is likely to be more relevant to your application.