http://www.jspwiki.org/Wiki.jsp?page=Hoop
Hoop is a similar to WikiGateway in that it exposes an XmlRpcWikiInterface for wikis which do not support the API themselves. It is written in Java.
The main difference between Hoop and WikiGateway is that WikiGateway tries to be more of a lightweight library.
Hoop provides caching, and requires integration with a web server. WikiGateway does not provide caching, and is installed as a CGI script. WikiGateway can also be used as a Perl module within another program (rather than as a web service over XML-RPC).
Also, Hoop is in Java and WikiGateway in is Perl.