It has not been maintained for current GATK, but this tool is designed to merge any non-variants from GATK 1.x all-sites output:
https://sites.google.com/site/gvcftools/home/configuration-and-analysis/gatk_to_gvcf-usage
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Thanks, Chris. What if I already have gvcfs with non-variant blocks, but want to use less stringent criteria to merge the non-variant blocks together?
Sorry, don't know of anything that would do this right out of the box..