National Archives of Guyana, 26 Main Street, Georgetown, Guyana, tel: (592) 227-7687, e-mail: firstname.lastname@example.org
General Register Office, GPO Building, Robb Street, Georgetown, Guyana, tel: (592) 225-7561
Hi Veronica. I am posting these two addresses for you and anyone else that would like to try writing. You will get a general response from the National Archives by email - and they will likely tell you to write to the General Register Office anyways. So, put pen to paper (sorry no email here) and send them your information.
You seem to have a lot of info already. You didn't say exactly what information you are wanting. From the GRO you can 'maybe' get a copy of your great grandfather's birth or baptismal certificate, his marriage license etc..There will be a charge for these but I haven't heard of anyone be successful in this endeavor. The GRO apparently has these records from 1860 onwards. Prior to that you'd likely be out of luck unless one of the many churches came through. You will likely find NO ONE cares about genealogy right now in Guyana and so, unless you have a contact right in Guyana you likely won't find much. There are a number of books on Guyana which on occasion mention names which may help.
Also put your surnames into Google you just never know what may turn up. Good Luck. Mrs. Davies