First create your mod. This will make the folder where your whole mod will be located.
First go into the Editor>Units>Players Menu
In here, we will clone the three wanderer classes. These will be the base of your class.
Remember to name your class properly, and change the path name.
In the "Gunner" base, remove all the Skills under the Skill tab. (These will not be needed)
Then change the Display names of the class, and the description. Do this for all three files.
Remember to save in the upper left corner.
This is the base for your Custom Class.

Go into the Data Editor>Skills
Go under the tab "Media>Skills" and Clone a skill you like the sound of.Remember to change the path accordingly.
Remove level 2-15 of the skill.
Change the requirements to be Guns and Cannons.
Make sure to save!

Go under UI in the lower left side of the screen, and press File>Open.
Open the Skill Tree you like the look of the most.
In the skill tree, click on the Rapidshot, and change the widget name to your spell name in the Properties tab.
After this, go into File>Save As, and save in the correct folder under MODS.
Go into the players menu, and go under you base class. Here you will change the Skill tree to your new skill tree.
Make sure to change you Hum_"Gunner"_F and Hum_"Gunner"_M's base file properly too.
Change the required level to 1, and the level to 1. Your starting ability should always have level set to 1, as this gives one skill point in it.
Make sure to save it everything once again!

In the Options menu in the lower right corner, change your Class to your class. Then restart GUTS. Then after restarting, you should press the play button.
We can see that our skill is in the skill tree, now to test the skill right click it.
Right click on the ground to shoot it.
Your class works! ... Kinda.

To pack the mod, you have to go to the lower left corner, and select the Mods menu. Click Mod. And then click Build to make your mod.
Congratulations! You have a fairly lacking class, but it is a class!