Well, crankycrank said to use a separate tool if the built-in one looks like it is distorting. Which is not bad advice, except that this particular extractor is beefier than the square taper ones it replaced, and the spline doesn't generally require as much torque to extract. So it is very unlikely to distort.

So I would not bother buying such a tool because I think the built in extractor is unlikely to fail, and removing the cap is going to be another thing that can get messed up. But GXP does use the same M22 crank threads and M15 crank bolt that Octalink and Isis use, so the oversize extractor for those should work fine.

However, larger spindle systems like DUB or BB30 are built just like GXP, but are much larger than M22. They have no Park remover tool and rely entirely on their built-ins for extraction. At the shop we used that extractor for every tune up. Same with GXP and most other self-extractors.

Personally, if the self extractor is the thin sheet metal type on square taper, and the extraction torque seems excessive, that's when I would use a separate tool. Otherwise, the built-in.