Can you elaborate on "it goes away"? How do you add the dependencies to the build path?

I add dependencies to the build path by right clicking the project -> Build Path -> Configure Build Path and added the external jars, javax.servlet.jar and portlet-api-3.0.0.jar. The error goes away temporarily but when I refresh the gradle, the added jars are gone and the error is showing again. 

Related issue :- When I build my jar, I get the following error 

Execution failed for task ':gradebook-service:compileJava'.
> Could not resolve all files for configuration ':gradebook-service:compileClasspath'.
   > Could not find javax.portlet:portlet-api:.
     Required by:
         project :gradebook-service
   > Could not find javax.servlet:javax.servlet-api:.
     Required by:
         project :gradebook-service

I have the following dependencies in my build.gradle 

    compileOnly group: "javax.portlet", name: "portlet-api"
     compileOnly group: "javax.servlet", name: "javax.servlet-api"