Gestalt, as a system function, allows you to get some information about the system environment. You provide a selector and Gestalt returns a 32-bit value, which has to be interpreted. You also get an error code (different from zero if problems are encountered).
The Gestalt function appeared in System 6.0.4. Initial version number is 1, and 4 or more on AV and PowerMac models.
A selector list found here covers all system versions from 6.0.4 to 7.5.1 version, QuickTime 2.1 version and several other external items from Apple.
In the list, lines in italics return an error code in which the corresponding selector is not available in the current system configuration. Lines in blue point out a selector which returns a value. This value is interpreted in the area above. If the selector returns attributes, possible 32 flags are displayed. Those with an arrow (and with blue display) have the value 1, those with no arrow (and black display) have the value 0.