It sounds retarded but the only way to beat GSP is... be better than GSP. Which essentially means becoming the best at EVERYTHING.
First off, and the most obvious one, is your wrestling needs to be insanely good. GSP is the very best of his division (Kos/Hendricks are right their with him), so to beat him, you have to become the best wrestler in your division... So basically the best WW MMA wrestler ever.
Now, if you use that wrestling defensively to keep the fight on your feet, well, you're going to have to have very, very technical striking with significant power too. GSP has some of the best striking defense of all the champs, statistically speaking and his jab is one of the best in the sport. So you have to get around dat jab, and then work your way through his tight defense.
If you want to use your world class wrestling to take it to the mat, then you're top game better be rock fucking solid, because GSP manages to reverse everyone or at least get back to his feet. And, of course, if he ends up on top of you, you have to be very good off your back too, because he will slice through your guard and make life very, very unpleasant.
Oh and you have to be an exceptional athlete too, because GSP is quick, strong and has cardio for days.
And he's one of the best ever when it comes to following gameplans.
Good fucking luck with all of that.
There's a reason he's the owner of arguably the best resume in the sport.