Listen bud, you can ask questions all day long about anything you want but when you're out there slinging aluminum through the air is not the time to be making it up as you go. My issues are this:
1. You should have known how to use a basic function of your GPS unit. I taught Garmins from handheld VFR units to the G1000 for several years and believe me, they are good for more than just hitting DIRECT.

2. You should have at least understood that when you file an RNAV equipment suffix (GPS is a form of RNAV) then ATC can and often does assign you direct to random points in space. If you wanted to fly VOR to VOR you should have filed the appropriate code.

3. You put a lot of blind faith in the guy sitting next to you. Just because he has 25 bazillion hours doesn't mean he is capable of ensuring that you are navigating correctly to a random point in space using nothing more than his experience and intuition.

4. When you realized you didn't know how to get there you just winged it. It worked for you this time because the geometry wasn't too different. What if you had been in Rockies instead of the plains of Texas? What if you were non-radar and there was other traffic nearby that you blundered into because you accepted a clearance, which ATC expected you to follow, but were incapable of following accurately.

5. Just because you weren't ever taught something doesn't mean you shouldn't know it. Crack open a book sometime. Namely the Garmin user's manual. If its in your airplane and operable you should know how to use it.

So I'm not just busting your balls for the fun of it. I'm busting your balls because you COULD have endangered yourself and others. Today you got lucky. Simple as that.