I have been told by 2 different pen kit providers that the gunmetal finish was developed? used? as a less expensive alternative to black titanium. They both indicated it had a durability like that of many of the "gold" platings - not gold titanium. As with all platings, durability tends to be a function of substrate (what is receiving the plating) preparation, the plating method used, the thickness of the plating applied - usually a function of time, and the composition of the plating. All of this is directly related to cost and ultimately price, but do not be deceived into believing all high priced plated components are the most durable.
If your goal is durability, you can test it for yourself. Buy a kit of interest; load up a white diamond buffing wheel, and buff a plated component, noting the time it takes for the plating to be removed from that component. Costly, maybe, but that must be weighed user/buyer expectation of durability - expressed or implied.
From strictly a hobby perspective, buy what you like; make the pen/pencil; use it; when plating material wears of or becomes too scratched up, discard it and make another - satisfying two things, the pleasure of making a pen, and the pleasure of using a pen you made.
