From the Introduction to the NIV Exhaustive Concordance [NIVEC], with some interspersed commentary:
Advances in biblical scholarship have made it difficult, if not impossible, to use Strong's century-old system. In the first place, Strong's system indexes only the vocabulary of the original-language texts that underlie the KJV.
This means some words in the Biblia Hebraica and even more in the UBS/ Nestle-Aland Greek NT are not covered by Strong's numbers.
Second, modern analysis of the biblical languages divides many words of identical spelling into two or three or more "homographs" - words with the same spelling but different meanings.
Strong did not have separate numbers for these look-alike words.
This is especially true of biblical Hebrew. Third, Strong mixed the Hebrew and Aramaic ("Chaldee") vocabularies together into one numbering scheme, whereas all modern lexicons and grammars treat them as different languages...
Finally, unlike the editor of the NASB Concordance who retained the Strong's numbers,
rather than attempt to patch up Strong's well-worn system, the editors of the NIVEC decided to make a clean break and develop a new standard for use in up-to-date biblical language reference works.
Despite the above claim, the new numbering system having been copyrighted by Zondervan, not everyone (esp. other publishers) seem willing to concede that it is as much of an improvement. Besides, Strong's system is in the public domain (therefore royalty-free as well as tweakable) and has a century of familiarity behind it. Thus we have Zondervan themselves publishing works that make use of the G/K numbers: The NIDNTT Abridged, Mounce's Greek and English Interlinear NT, NIDOTTE. An exception is Spiros Zodhiates' NIV Hebrew-Greek Key Word Study Bible (AMG Publishers).