Can I use garlic leaf for cooking?
When we have had garlic in our garden I have used the garlic leaves. They do have a garlicky flavor but are milder than garlic cloves. I tend to use them more as I would chives or garlic chives as in addition to having the milder flavor than the cloves they make for a quite nice presentation.

Regarding drying them, I have never tried it. Off the top of my head I can't think of any reason not to dry them for later use but there may be issues that I just don't know about.