The way the system displays rank is based on the xp table from the rule book, and in brackets is the fighters class upon creation. For example, a fighter who has 'mighty ganger (juve)' means the fighter currently has the rank of mighty ganger, but started life as a juve, or again 'gang hero (Leader)' means the fighter started as the Leader and has the rank of 'gang hero'.
There currently isn't a marker to designate a fighter as the Leader (unless there is; I browse on my iPhone so some of the functions may be unavailable to me). Generally the leader will be at the top of the roster, and I believe you can drag and drop the fighters into a specific order (I can't do it from my iPhone but I seem to remember it being stated this is possible).
You may notice in the ganger edit screen below the xp value there is a field next to an eye icon. This allows you to give a three letter (or number) designation for a fighter, so you could enter [LDR] here or something to denote that fighter as the leader.
Many moons ago there was talk of a drag and drop badge to designate the Leader but it may not be too high in the mighty Malo's to-do list as he has other more pressing matters what with single handedly coding this whole thing and setting up tools for the other SG's on the site.
The system doesn't actually automatically add anything to a fighters skills list if they're created as a leader so as far as the yak system is concerned the distinction is purely academic; though your new leader will inherit the perks of being a leader (such as the ability to unpin early and the ability to wield specialist weapons), these are inherent to the fact that the guy is the Leader rather than individually listed skills, so no book keeping is required in that regard.
And as
@spafe has said the xp remains the same, and the fighter does not gain the basic stats of a leader but they DO inherit the traits of being a leader like specialist weapons, unpinning early, and leadership conference to other fighters within range.
Hope this helps!
[EDIT]
As the primary issue here relates to the workings of the Yaktribe gang management system I have moved the thread to the relevant forum. Though the follow up question is related to game mechanics, this issue has been asked and answered elsewhere so I have disregarded it in terms of thread classification.
