How do I sort entries by two variables?
If you want to order by multiple columns that include custom fields, first make sure you're at least on Craft 2.4.2684, then you'll be able to use this syntax:

{% for gallery in craft.entries.section('Gallery').order('postDate desc, available desc').limit(null) %}


Also note that that you should be using limit(null) instead of limit(none) if you want to result all results.