I need help

I'm trying to trace records of an ancestor who was a convict sent here to Australia.
His name was Gilbert Marshall and he was born around 1800 and from records of his transportation and some newspaper article from the Bristol newspapers from the time (supplied to me from a lovely member here) I've been able to see that he was convicted in Bristol (city) on the 3rd April 1819 and originally sentences to death which was then changed to transportation for 14 years. it also mentions Newgate prison and that the case was seen as a quarter session. I'm trying to find any reference to his parents names or any siblings that he might have had, i want to trace this tree back further but I'm stuck on him. If anyone can help in any small way or point me in the right direction i would really appreciate it.
regards
Melissa