Gibberbot does not show offline contacts. However, it is standard practice for some to appear offline, while actually remaining online. Most XMPP clients support initiating conversations with apparently-offline contacts. Gibberbot should support the option to display offline contacts, and this should be editable on a per-contact basis.
For instance, Alice may always want to see all her contacts, so she sets the 'show offline contacts' option in the settings. Alice's interface shoes online contacts at the top, and offline contacts below them in the list.
Bob, however, only wants to see people with whom he may be able to converse. He knows that Alice rarely appears offline when she's actually online, but that Charlie is almost always available for urgent communication, even when he appears offline. Bob edits his settings for Charlie, checking the 'show in the contact list even when offline' box. Bob's contact list always shows Charlie, whatever his status, but when Alice is offline, she does not appear in Bob's contact list.