Simple English translation options
Among the top 10 bestselling Bible translations in 2020:
- NIV (1978, latest update 2011) ranks #1
- KJV (1611, latest update 1900) ranks #2
- NLT (latest update 2004) ranks #3
- ESV (2001, latest update 2016) ranks #4
- CSB (2017, revised 2020) ranks #6
- NIrV (1996) ranks #8
- The Message (2002) ranks #9
- GNT (1976) is not in the top 10
Judging by the chart below (source), and from Logos Bible Translation Spectrum wiki page, from most literal to paraphrase:
- ESV: word-for-word, "essentially literal"
- KJV : word-for-word
- CSB (major revision of HCSB) : thought-for-thought, "optimal equivalence"
- NIV : thought-for-thought, "balanced approach"
- NLT : thought-for-thought, dynamic equivalent
- GNT : thought-for-thought, Eugene Nida's method "functional equivalence"
- The Message : paraphrase
As for readability as US school grade level, BibleGateway and christianbook.com ranks:
- KJV: 12
- NIV, CSB, and GNT: 7
- ESV: 10
- NLT: 6
- The Message: 4-5
- NIrV: 3
Based on the comparison above, to fit your criteria of most widely used "simple English" for "unchurched" people, and taking into consideration accuracy, I recommend 3 options: NLT, CSB, then NIV. But as a replacement for GNT, I think NLT is your best choice since it is the most readable. Although The Message is very readable, it's not recommended, since it is a paraphrase (not that accurate). Although KVJ ranks #2 it is requires too high a reading level.
NLT (New Living Translation): NLT has also been gaining popularity (move to rank #3 from rank #4 nine years ago) and there are more packaging choices as well as more study Bible with NLT translations compared to the GNT (Good News Bible). In a 2018 Bible Gateway blog guest post The Mission and Impact of the New Living Translation Bible, Mark Taylor (CEO of Tyndale, publisher of NLT) made the case for NLT, which grew out of The Living Bible which his father published in 1971.
Christian Standard Bible (CSB): more accurate than NLT, ranking #6 in sales and only slightly higher reading grade level. According to a report from Global Bible Initiative CSB has the best overall score (70.3%) when balancing readability and literalness compared to ESV (69.3%), NIV (63.6%), KJV (61.0%), NLT (56.6%), etc. (GNT is not included in the study).
- For Literalness, ESV is 68.74%, KJV is 66.58%, CSB is 64.83%, NIV is 53.10%, and NLT is 39.90%.
- For Readability, NLT is 70.08%, NIV is 67.20%, CSB is 66.75%, and KJV is 48.83%.
NIV : similar reading level with CSB, but less literal than CSB. Since it has been around the longest, has the most selection of editions and study Bibles.
Study Bible for beginners
For "unchurched people" I also recommend a Bible embedded inside a beginner-level study Bible such as the NLT Life Application Study Bible, 3rd Ed (2019), winner of the 2020 Christian Book Award for Bible of the Year (Evangelical Christian Publishers Association). Purists may object to the unavoidable denominational perspective of a study Bible (I try to research the bias of the NLT Life Application), but I think it's more important that a study Bible can sustain their actual reading of the Bible via its user-friendly book intro & commentaries, and catchy side notes.
There are several options for study bible for new believers reviewed at The Best Bibles for New Believers (written in 2020), recommending the New Believer's Bible NLT published in 2020 with special features tailored for new believers. It's edited by Greg Laurie, founder and senior pastor of a non-denominational mega church Harvest Christian Fellowship which in 2017 joined the Southern Baptist Convention which should give you a clue of the possible theological bias in the study notes.
CSB also has a growing number of study bibles available.