I'm continuing to research the GATCUMs and have a tree back to 1700 - when the family were in Chilbolton, Hampshire - right through to the present day. Apart from a few gaps, I have information on most of the branches which I am happy to share.
Spelling variants I've found include: GATCOM(e), GADCOM(e), GADCUME and occasionally GATCOMB(e) although the latter appears to 'belong' to another family group which has its roots in Somerset.
If you would like to contact me off-list, my email address is:
joy2811uk at yahoo.co.uk
(written this way to discourage spammers! ... just close the spaces and use the @ sign)