why gateway IP ?

On our network we don’t have any windows domain model. all the clients are running on WIN 98.

we have squid proxy running on linux. am assigning this proxy on all the client machine. they are able to browse , but they are not able to use citrix & ftp clients. since i have not assigned any gateway ip address in their network properties. it works when i give the gateway ip address. but why can’t these citrix & ftp clients work with the existing proxy server. why do they need gateway ip address entered in network properties. how do i make these clients to work well only with proxy server.