Ascii formatting characters deviate from textile formatting
The textile shortcuts which the Gajim ascii formatting is, presumably, based on use the underscore character to denote emphasis (e.g., emphasis) while Gajim uses it to underline text and uses a forward slash character to denote emphasis.
See this textile shortcut reference. http://rpc.textpattern.com/help/?item=textile_comments
To complicate matters, Google Talk and perhaps other programs use the textile shortcuts as they were intended so users of Google Talk communicating with users of Gajim will see different formatting. There are also many complaints about the forward slash character disrupting code snippets in conversation.
The formatting shortcuts should be changed to match textile and Google Talk.