If you go to https://explore.dgnu.org/ and use the --version switch, it displays
gdc (GCC) 9.0.1 20190425 (experimental)
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Compiler returned: 0
The downloads from GDC - D Programming Language for GCC use a D compiler frontend which is based on DMD 2.068.2, which dates back to Sep 23, 2015.
gdcproject.org/downloads/LATEST is wrong and the downloads on GDC - D Programming Language for GCC are not the latest you could find… probably related to GDC being merged into GCC?
So it would be real nice for the D community to have a more recent GDC in Travis.