Unfortunately you didn't specify a particular time period regarding Gibbs in Guyana, but here is what I've come up with.
In the 1822 Guiana Chronicle and Demerara Gazette there is frequent mention of a mercantile house called McMahon, Gibbs & Co. This is the only year so far that I have come across the combined names. I believe that the above McMahon to be the brother of my 3 x ggrandmother, hence my interest.
An index of BDMs in The Colonist Newspaper, Georgetown, British Guiana for 1864-1880 has the following entries: Gibbs - 1870
An index of BDMs in The Argosy Newspaper, Georgetown, British Guiana for 1880-1896 has the following entries: Gibbs - 1884
Copies of these newspaper are to be found at The British Library Newspaper Library, Colindale Avenue, London NW9 5HE England and also at the Public Record Office, Kew, Richmond Surrey TW9 4DU England
Regret I have no further information.