Spec for GPAC's DashCast configuration? According to DashCast, we should provide a configuration file of several entries. The document demos some options:

[v1] 
type=video 
width=1280 
height=720 
bitrate=800000 
[v2]
type=video 
width=640 
height=360 
bitrate=400000 
[a1]
type=audio
bitrate=128000
[a2]
type=audio
bitrate=256000


For my first run of DashCash, the configuration file would be automatically generated if it is not existing. Here is the autogenerated one:

[v1]
type=video
bitrate=2000000
framerate=24
width=1920
height=1080
crop_x=0
crop_y=0
codec=libx264

[a1]
type=audio
bitrate=192000
samplerate=44100
channels=2
codec=aac


I'm wondering where I can get a spec of options for this configuration file?
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.