There is a standard answer and a more cynical answer. Glassy carbon (GC) has very slow electron transfer kinetics for most inner sphere processes (e.g. redox chemistry of many analytes and interferences), especially if it is clean and has very little oxygen functionality. Additionally, most species do not adsorb on it. Conversely, gold is a good electrocatalyst for many processes, especially oxidation of a wide range of biomolecules. 

So because of the above, this means GC gives you a larger electrochemical window in water. Also, the electrochemistry of the analytes and the interferences can be difficult to see/shifted to relatively high overpotentials. Therefore, when you successfully modify the surface (e.g. the GC is your substrate electrode) you can see a big difference. The lack of electrochemistry occurring at the GC is also important if your modifier is selective, because the GC is much less likely to interact with interferences than a gold electrode substrate.

A cynical interpretation of this is that GC is one of the worst substrates for electrochemically oxidising a range of common analytes. Therefore as soon as you do any modification to a GC substrate, it is easy to make the electrochemistry look much better. The modification can be easily proven to be 'effective', e.g. by looking at the sensitivity and limit of detection for the GC vs modified GC. Conversely, because a gold electrode can be so effective, many modifications using gold as a substrate electrode would result in performance remaining unchanged, or even getting worse (which cannot be published).

Some might mistake GC as a cheaper substrate than gold, but this is simply not true. The very impressive mechanical properties and chemical resistance of GC is a major advantage for an electrode in a lab, but a major disadvantage in using it in any kind of scaled-down and mass produced electrochemical sensor. 

There is a standard answer and a more cynical answer. Glassy carbon (GC) has very slow electron transfer kinetics for most inner sphere processes (e.g. redox chemistry of many analytes and interferences), especially if it is clean and has very little oxygen functionality. Additionally, most species do not adsorb on it. Conversely, gold is a good electrocatalyst for many processes, especially oxidation of a wide range of biomolecules. 

So because of the above, this means GC gives you a larger electrochemical window in water. Also, the electrochemistry of the analytes and the interferences can be difficult to see/shifted to relatively high overpotentials. Therefore, when you successfully modify the surface (e.g. the GC is your substrate electrode) you can see a big difference. The lack of electrochemistry occurring at the GC is also important if your modifier is selective, because the GC is much less likely to interact with interferences than a gold electrode substrate.

A cynical interpretation of this is that GC is one of the worst substrates for electrochemically oxidising a range of common analytes. Therefore as soon as you do any modification to a GC substrate, it is easy to make the electrochemistry look much better. The modification can be easily proven to be 'effective', e.g. by looking at the sensitivity and limit of detection for the GC vs modified GC. Conversely, because a gold electrode can be so effective, many modifications using gold as a substrate electrode would result in performance remaining unchanged, or even getting worse (which cannot be published).

Some might mistake GC as a cheaper substrate than gold, but this is simply not true. The very impressive mechanical properties and chemical resistance of GC is a major advantage for an electrode in a lab, but a major disadvantage in using it in any kind of scaled-down and mass produced electrochemical sensor. 

 Dear Leigh Aldous

Thank you very much for your answer

Apart from cost? GC is a highly variable material and really difficult to pre-treat in a way that leads to reproducible behaviour in ticklish situations.  Electrochemical pre-treatment in acid generally perks it up for inner sphere reactions by introducing more oxygen functionality on the surface, but if potential excursions have been above about +1.2 V to Ag.AgCl/ 3M KCl then quinone moieties lead to surface redox behaviour at potentials you'd ordinarily hope were in the double layer region. Anywhere between 0 V to +0.5 V can feature this complex and pH-dependent redox. The redeeming feature is the ease of functionalisation and massive range of chemistries that can be employed. For gold you're more or less stuck with thiol SAMs.  These are a lot less stable than generally supposed and can exchange with thiols from solution, even at neutral pH and away from reducing potentials.  The only other modification chemistry that works on gold is the reduction of diazonium compounds.  On carbon, you can do the diazonium reduction, amine oxidation, use conventional EDC coupling between carboxylate groups on the carbon and amines, pretty much all of your first two years of study of organic chemistry can be usefully employed.  Finally, GC in its pristine state shows very low electrocatalytic activity, so O2 reduction presents less of an issue and you may get better solvent limits and less interference in specific cases. Greg Swain has written extensively about ways of preparing GC reproducibly, including a very nifty trick of polishing in degassed cyclohexane to ensure low O functionality and establish base lines.  Other key players in this field include McCreery who published some very useful protocols in a series of Anal. Chem. papers on how to characeterise any surface pretreatments.

Dear Danny O'Hare

Thank you very much for your answer