How can I measure the gap by my self

Those are millimetres, right?
(Not everyone has caught up to the 18th Century yet.)

Unfortunately, inches wouldn't be easy unless you have machinist level tools. 
0.026
0.025
But they are both off by a few 10thou so if you used inch gauges and converted to mm, you wouldn't be able to line them up with published specs. 

Luckily I was educated in the metric system, millimeters is something I understand, I would be lost with inches, my feeler gouge is in mm.

You have it correct and the published specs are in mm. However, I was replying to another poster who seemed to want inch specs because I assume he only has inch feeler gauges. My post was intended to bring up the problem with trying to use inches for something published in mm.