Because GnuPG is open source, it's been extensively peer reviewed and found safe and secure. That doesn't mean it's perfect and has no errors. But they are much less likely to exist in GnuPG than in some other solutions; particularly proprietary ones.
Are there more than 3 current OpenPGP tools? 1) GnuPG, GPL'ed open source, based on GnuPG's own libgcrypt family of libraries. Many many features, including NSA SuiteB support. Widely used in scripts, relied on by Thunderbird EnigMail, and other tools. 2) NetPGP, BSD'ed open source, depends on libOpenSSL, and it's own OpenPGP:SDK (C library). Basic features only, more like last pgpi.org PGP 2.x open source command line tool. Very few ports, besides NetBSD (NetPGP's sponsor). less peer review than GPG. No NSA SuiteB support (though libOpenSSL does support it). Someone needs to add SuiteB support, and a few more ports, support for opensource keyservers, and SuiteB, then it would be a nice option. 3) PGP product Symantec/PGPcorp. extremely expensive, closed source, patented keyserver tech, zero community review. Apparently a rich set of features for commercial enterprise use. If there are other open source OpenPGP tools besides GnuPG and NetPGP, that would be welcome news.