I'm going to give you realistic numbers:
Your school determines a COA (which includes your tuition for fall/winter/summer). Let's say it's 85,000.
Your Direct Unsub loans is determined by your COA and what your family can contribute.
Let's say your Direct Unsub loans are 63,000.
In order to make up the difference between your COA and what Direct Unsub gives you, that's where the Grad PLUS loans come in.
The maximum you can borrow from Grad PLUS would be 85k - 63k = 22k.
You can't apply for Grad PLUS until April 1st. But keep in mind they do a credit check. And sometimes you'll need a co-signer.
Also keep in mind the COA is an overestimate. You need to borrow money up to the 85k, but if you have more money than what's needed, then give it back to the loan service after the school year is over.
To answer your original question, the answer is Yes. You're borrowing all the money that you need for one school year (including Summer I).