A working solution is to specify the reference line with a full ISO 8601 string, ie "YYYY-MM-DDTHH-MM-SS.sss": "2020-06-15T00:00:00.000". Apparently the GPL is quite picky about parameters...
I also changed the line size and shape.
HTH /PR
https://en.wikipedia.org/wiki/ISO_8601GGRAPH
/GRAPHDATASET NAME="graphdataset" VARIABLES=Date Growth
MISSING=LISTWISE REPORTMISSING=NO
/GRAPHSPEC SOURCE=INLINE.
BEGIN GPL
SOURCE: s=userSource(id("graphdataset"))
DATA: Date=col(source(s), name("Date"))
DATA: Growth=col(source(s), name("Growth"))
GUIDE: axis(dim(1), label("Date"))
GUIDE: axis(dim(2), label("Growth"))
GUIDE: form.line(position("2020-06-15T00:00:00.000", *), shape(shape.dash), size(size."2px"))
GUIDE: form.line(position(*, 0.2))
ELEMENT: line(position(Date*Growth), missing.wings())
END GPL.