Actually, "Green Glue" is just a brand name. It, and other specialized damping glues are "viscoelastic" and are used as "constrained damping" materials. These material's damping performance results from their being constrained/compressed between two rigid surfaces. This is quite different from, and should not be confused with "limp mass" type products such as compressed vinyl sheet block sound barrier or high density rigid insulation materials.
All of these products regardless of type are part of a wall construction "system". In other words, how the wall is constructed is of primary importance in how much sound transmission loss is achieved and at what frequencies. In building construction this is referred to as "Sound Transmission Class" or STC rating.
While adding more mass is generally considered a good idea for reducing sound transmission, there's more to it than that, especially with the newly developed materials we have available today.
National Gypsum's SoundBreak XP drywall is not just a single layer higher density (higher mass) drywall. It, like CertainTeed's SilentFX are both double layer high density gypsum board with a layer of viscoelastic constrained damping material sandwiched between.
The performance of either of these products will largely depend on the construction of the wall itself. The best results are achieved when either manufacture's products are used with a resilient channel which decouples the drywall from the structure (studs, joists). The Green Glue product line also has their own specialized version of this which they call a noise proofing clip. Care must be taken not to have the wall/ceiling SoundBreakXP/SilentFX panels come in contact with surrounding surfaces and the gaps must be sealed with a noise-proofing sealant, not just regular caulk. Both products are most effective when used in multiple layers with a viscoelastic constrained damping glue sandwiched between each layer, about 3 tubes per 4'x8' drywall sheet for maximum performance.
In order to achieve a high STC rating you must follow the manufacturer's specification.
I am currently in the process of implementing these products into my existing purposed built listening room which was built in the early 1980's. I wish these products were available back then.
Because of the convenience of ordering, I'll probably use National Gypsum SoundBreakXP because I can order it from Menard's in smaller quantities (so I can tackle this in several smaller projects) and they have more sheet sizes, Green Glue and Green Glue Noise Proofing Clips (attached to the inside wall of the existing room walls).
Just for the record, I'm an architect by education although I've left the profession in search of "greener pastures".