Spec for GPAC's DashCast configuration?
This is available on the page you linked to directly.  Look under the Configuration Files section.

Relevant details from the page:

A file "-conf" file.

[ID]        # The ID of the representation
type=T      # T can be "audio" or "video"
width=W     # if type is "video", 
            # W is the width of the representation
height=H    # if type is "video", 
            # H is the height of the representation
bitrate=B   # B is the bitrate (bps) of "video" or "audio"


And a "-switch-source" file.

[ID]         # The ID of the source identification.
type=T       # T can be "video"
start=ST     # ST is the start time YYYY-MM-DD hh:mm:ss
end=ET       # ET is the end time YYYY-MM-DD hh:mm:ss
source=S     # S is the address of the source


Additionally, from looking in the source itself, the configuration file is read in the cmd_data.c file starting on line 160 or so (depending on rev) in the dc_read_configuration method.

For the conf file, video type can have codec, framerate, crop_x, crop_y and custom.  Audio types can have codec, samplerate, channels and custom.

Custom appears to push the value assigned directly in to the libav codec.

For the Switch source file, the type appears to actually be able to be video or audio.  Both only have the listed 4 options.