For a high grade glioma in an 82 yo, which treatment is better, gamma knife or stereotactic radiation therapy?

Depends on size: Gamma knife and stereotactic radiosurgery (srs) are terms that are used interchangeably. What it refers to is focused beam radiosurgery. Gamma knife is just a particular platform. Srs may be delivered by gamma knife, novalis, cyberknife. Usually for gliomas, they are large and srs is not used. Conformal radiatherapy covers a larger volume and is fractionated over time.

Stereotactic : Radiosurgery (srs), whether by gamma knife, cyberknife, or any other machine is not standard treatment for glioma. Glioma infiltrates into normal brain and so a generous "safety margin" of brain outside if the tumor or cavity must be treated. Conformal daily rt over many weeks safely treats the tumor infiltrating the brain. Srs cannot safely do that, but is sometimes used if standard rt fails.

Neither: In reality, , a high grade glioma does not have sharp edges by definition and these methods have sharp edges by definition. If you only want to palliate, invoked such costly options (both have high billing codes/prices), simpler 3d conformal therapy may be more than adequate. This topic id debated and data are variable. A tiny lesion might be able to handle these options.