The GM Tool was designed to support (virtually) any roleplaying system. It is not intended as a character creation (or editing) tool, just as a possibility for the gm to manage the characters, view the stats and (later) organize combat.
A character for the GM Tool is a basically a simple textfile, that describes the character. Every character consists of a “Nodes”, which can have a value (a text) and/or sub-Nodes (which again can have a value and/or sub-Nodes, etc.). Normaly, it's totaly up to you, what Nodes you want to use. Simply use, what works best for you and your gaming system. If you want to use custom character sheets, you will have to use the Nodes that the custom sheet demands, otherwise it will not work correctly. Custom sheets should always come with a description of the used character format.
My sample character Race=Human Job=Peasant Attributes.Strength=10 Attributes.Dexterity=10 Attributes.IQ=10 Attributes.Health=10 Skills.Combat.Sword=12 Skills.Combat.Shield=11 Skills.Social.FastTalk=10
You can copy this character into a text file and try to load it: It will work. You can edit the file extension for character files in the “Options” tab, btw.
So, how does it work? The first line is always the name, that simple. All the other lines have a “node” and a value, seperated by the ”=”. So, for example “Race=Human” creates a Node “Race” with the value “Human”. “Attributes.Strength=10” creates a node called “Attributes”, adds a sub-node “Strength” to it and sets the value of this subnode to 10. Later, “Skills.Combat.Sword=12” creates a node “Skills”, adds a sub-node “Combat” to it and a sub-node “Sword” to “Combat” - and gives the value “12” to “Sword”.
You can of course add a value AND sub-nodes to a node, if you add, for example, “Attributes=Bla” to the example above, the “Attributes” node would not only have 4 sub-nodes, but also the value “Bla”.
You can have as many nodes and sub-nodes as you want, so that you are able to describe any character from (virtually) any system.