W_B;

I think what the "mask" is refering to is the frame around the gauge, called the Bezel. Normally, they're simply drawn in a paint program in an 8 bit, 256 color format, and used with the gauge.
It is possible to "borrow" one from another panel and edit it to change the color or texture. But, this must be done in whatever paint program you use.

The x, y numbers are referenced at the bottom of the FS Panel Studio, or CfgEdit window. These are the coordinances that the program plots to show where on your panel a gauge is placed. Normally 2 numbers, x=vertical placement and y=horizontal placement. (Hopefully, I don't have that backwards).
I think your W, H, numbers are the width and height of your gauge, in pixels.
The third number is the size of the gauge, as it's displayed on your panel.

And, I'm not sure what you mean by "callback". I can't say I've ever seen that.

What panel editor are you using? I use FS Panel Studio.