Ho Creek!

To answer your questions:

1) Yes, but if your touch/screen stylus does not act like a digitizer (pressure sensitivity) it will only act like a mouse (i.e. when you drag, it will draw) without the extra features like changing width based on pressure or pen tilt.

2) You can either drop in images as new layers in Gimp and build a map up that way, or create/use a gimp brush or animated brush (hose) to "stamp" down structures or vegetation.

3) Not many, but you can find a few through search, and it depends on the style of map you hope to make:

http://www.cartographersguild.com/tu...easy-gimp.html
http://www.cartographersguild.com/tu...owns-gimp.html

or a few in Photoshop that can be used as rough guides with pretty good success:
http://www.cartographersguild.com/tu...-tutorial.html
http://www.cartographersguild.com/tu...photoshop.html
http://www.cartographersguild.com/tu...-gullside.html

-Rob A>