You can do this as a cell array of chars, or you can do it with an actual char array if you really must. Another option would be a string array, if that works for your needs.
S = load('matrix_double_3D.mat');
G = S.GPS_reference_PRN_double_original_epochs_3D;
% as a cell array
Gcl = num2cell(G);
Gcl = cellfun(@(x) sprintf('G%d',x),Gcl,'uniform',false);
Gcl(1:3,:,1:3) % show a sample
% as a fragile char array
% assumes absval of numbers are less than 100
Gch = permute(reshape(sprintf('G%02d',G),3,size(G,1),size(G,3)),[2 1 3]);
Gch(1:3,:,1:3) % show a sample
