Both of them are different.
HttpFS is a server that provides a REST HTTP gateway supporting all HDFS File System operations (read and write). And it is inteoperable with the webhdfs REST HTTP API.
The Apache Knox Gateway (“Knox”) provides perimeter security so that the enterprise can confidently extend Hadoop access to more of those new users while also maintaining compliance with enterprise security policies.
If you want to access HDFS via Knox , then you can do it using webhdfs or httpFS.
This article provides detailed procedure of configuring Knox with httpfs.
This doc provides configuration for Knox with webhdfs
Additional resources : https://community.hortonworks.com/articles/18075/comparison-of-httpfs-and-webhdfs.html