The GNU Guix project invented G-Expressions to make it easier to "stage" data or code, in the form of S-Expressions, for later manipulation or evaluation. They are similar to S-Expressions, hence the name, but they provide useful code staging features beyond what can be easily accomplished with just "quasiquote" and "unquote". A high-level object (such as a Guix package) can be included in a G-Expression; the transitive dependencies of that high-level object will then be automatically carried along with the G-Expression. When the G-Expression is converted to an S-Expression and stored on disk for later manipulation or evaluation, the high-level object will be automatically "lowered" to an appropriate representation (such as the package's output path) via a "compiler". Compared to direct manipulation of S-Expressions, G-Expressions can provide a simpler and more intuitive way to stage data or code.
The Guix project uses G-Expressions to accomplish a wide variety of tasks, including:
Building the latest version of Guix via "guix pull"
Executing the "liberation" procedure to convert Mozilla Firefox's source code into GNU IceCat's source code
Building Docker containers from scratch
Executing activation actions during system boot