You do not NEED to use gesso. When I first started art journaling I did not use it. My pages turned out just fine. Over time, I found the gesso helps stop the the paints/inks/water whatever I'm using from seeping or staining the back of the page. With gesso, I can use whatever media I want on my page without worrying the colors will run or stain the preceeding page.
You can probably use whatever you've been using in stanmping for art journaling . As someone mentioned earlier, just use with you are already comfortable with. In fact i think I use my stamping stuff more on art journal pages that I did when I was strictly stamping. The only thing I haven't figured out is how to use my cuttlebug on bound journal pages, but I will get there!
There area LOT of great resources on the web- ideas, online classes, blogs, etc . It is so eay to get overwhelmed. The best thing you can do is just CREATE. There is no "wrong" in art journaling. If you dont like what you've done you can paint, collage or gesso over it ( another great use for gesso!)