smart+question
Currently, Gajim does not allow a contact, that is in a group to be removed from that group if it was the last one. XMPP IM, section 7.1 states that grouping is optional, so it should be possible to remove a contact from all groups.
Ungrouped roster items are now shown in a 'General' group, although that group doesn't really exist. Among different clients this is the case, but the name for the group is usually different. Examples are 'Unsorted', 'Unfiled', etc. However, most of these clients (like Gajim) actually allow items to be added to a group with the same name. When switching to other clients, you will end up with multiple of these groups, which is irritating and confusing.
My suggestion is to not show the 'General' group in the 'Edit Groups' dialog, unless such a group really exists. You may also want to consider using a different way of showing the 'group' of ungrouped items. For example by using another font or color for the 'group' heading in the roster to differentiate between ungrouped items and items in a group that is actually named 'General', by coincidence. I suppose the latter also holds for the 'Transports' 'group'.