G_code{1,1}{:} is a comma separated list (CSL), which is not - I guess - what you thought it was. It is not possible to tell you what is wrong without knowing what you store in G_code and how you want it written in the file, but to illustrate, if G_code{1,1} where the cell array {'hello', 'world'}, the following line
fprintf('%s %s\n', G_code{1,1}{:}) ;
would be equivalent to
fprintf('%s %s\n', G_code{1,1}{1}, G_code{1,1}{2}) ;
or taking the content
fprintf('%s %s\n', 'hello', 'world') ;
So you see that {:} returns a list of cells' content separated by commas.
EDIT: as FPRINTF repeats its formatSpec until all its args are displayed, you can use CSL the following way: assume you want to have two columns in the output with the data
>> data = {'A1', 'A2'; 'B1', 'B2'} ;
you can specify a formatSpec with two column, and develop data as a CSL (but just once)
>> fprintf('%s %s\n', data{:}) ;
A1 B1
A2 B2
which is equivalent to
>> data_col = data(:) ;
>> fprintf('%s %s\n', data_col{1}, data_col{2}, data_col{3}, data_col{4}) ;
