Hi Keith
You will only find out about his parents by ordering the birth cert for your grandfather. There is only one entry showing on
www.scotlandspeople.gov.uk There is only one Peter Gibson Clark showing on SP, so happens the details all match what you have so a sure bet it is likely to be his birth entry.
Scottish birth certs will include full details on parents' names and also date and place of their marriage to help you work back.
Regarding possible siblings, this is where it gets tricky on RootsChat. Possibility some siblings could still be alive and living relative details are not allowed to be posted here on RootsChat. If someone does recognise his name, the best (and only) way to contact you would be via the PM service here on RC.
Regarding working back on his line, ordering his birth cert would let you start to do this

Monica