Are there open-sourced graph-based variant callers?
Yes, there are. There are some suggestions in the comments (VG, WhatsHap, GraphAligner, Minigraph).
However, to be clear, the current default variant calling algorithm used in the GATK (the haplotype caller) is a graph-based algorithm. It constructs a De Bruijn graph from mapped reads for local assembly around variants. Other tools that also follow this strategy include Platypus and freebayes.
You might also consider multi-color genome assemblers. For example, Cortex and Corticall use colored De Bruijn graphs to aggregate reads from multiple genomes and call variants.