I've got a Surface Pro 3, I'm just wondering if anybody knows of any games on GOG that are touch-screen or pen-input friendly?

I believe the Wadjet Eye games were released for mobile platforms as well, so I'd expect they're touchscreen friendly.

I think Tales of Monkey Island was touchscreen friendly as well (much to the detriment of playing it with a mouse and keyboard).

Heck, a lot of adventure games with persistent onscreen controls (Indiana Jones and the Fate of Atlantis, Monkey Island oldstyle, etc.), could be considered touchscreen friendly if you don't mind the lack of "hover" for the mouse (to be able to tell what the mouse is over at that point).

When my tablet was new I played most of the game 'manually'. But I got annoyed, because I had to doubled click each time I performed a left click. So I bought a kind of reading tray (with a cushion filled with grain underneath the wooden tray), placed it on my legs and use a mouse. And there is also a little space left for a glass of wine...

If you have a way to get a virtual keyboard, any turn based game should be playable (though not ideal) on a tablet.

In the case of a naughty developer who puts an action sequence into an otherwise turn based game, you can hook up USB input devices to get past that part. (Note that I consider this bad game design on the part of the developer.)