The GATK output vcf file contains genotypes in which alleles could be separated both by vertical bar ("|") and slash symbols ("/"). For example: 0/1, 0|1, 0/0, 1|1.  I found the explanation at [EBI][1] portal. It is said that

>  The vertical pipe | indicates that the genotype is phased, and is
> used to indicate which chromosome the alleles are on. If this is a
> slash / rather than a vertical pipe, it means we don’t know which
> chromosome they are on.

Does this statement correct for GATK output?

  [1]: https://www.ebi.ac.uk/training/online/course/human-genetic-variation-introduction/exercise-title/want-know-how-we-did-it
>Does this statement correct for GATK output?

The very page you linked says so. This is part of the [VCF specifications][1], so all correctly formatted vcf files should abide to it:

 - /:  genotype unphased
 - |:  genotype phased


  [1]: https://samtools.github.io/hts-specs/VCFv4.2.pdf
  [2]: https://samtools.github.io/hts-specs/VCFv4.2.pdf