Can I do sub-forms?

It's not built in. There are a couple ways you could approach it.

One would be to collect data in form one, then pass that to form two. After form two, you could send them to form three, passing the data along there as well.

You could also use conditional logic to reveal groups of fields based on a previous selection. If the participant had 4 entries, you could have them fill out their author information, then make a selection from a group of radio buttons, and then reveal that many sections (one for each entry.) This option ends up being a lot of work because you have to have to create one entry section for one entry, two sections for two entries, three for three entries. If you allowed up to 10 entries per form submission, for example, you'd have 55 'entry sections' in the form. That would be painful.

I think I would focus on passing the data from form 1 to form 2, and then 3, 4, 5, etc.

There is no provision in Gravity Forms to have nested forms. The closest built in would be conditional logic. But I think I would approach it as multiple submissions of the same form, passing data from the first entry to all subsequent submissions.

The Gravity Forms PayPal Add-on is not designed to work as a full shopping cart. It may not be ideal for what you want to do. I'm having a hard time envisioning how this is supposed to work on your site. Do you have a link to this type of functionality?