NOTE: This is roughly obsolete, but may be helpful for historical purposes later. The issue is covered more precisely in XEPs and ClientIssues (though no mention of clients that do support jabber:iq:gateway is made - may want to add that eventually).
Is the user likely to have this? Here's a survey (see also some historical details, summarized below):
- Conversations - not supported
- Gajim - supported
- Psi - supported
- Movim - supported