Our goal was to have enough Grbl compatibility to allow use of gcode senders designed for Grbl. This is for the gcode sender (running a job) portion only. We did not try to make it compatible with any setup wizards that some senders have.
FluidNC implements the GRBL line protocol per Classic Grbl Line Protocol and serial_protocol. As with Plain Old Grbl, commands are sent line-by-line, with flow control via acknowledgments of either "ok" or "error...". There are a few single-character "realtime commands" that are not line-oriented; they are used for immediate actions like status requests, program pause/resume, and overrides.
In addition to Grbl's status report requests via ?, $G, and $#, FluidNC also has an optional Automatic Reporting feature
FluidNC has quite a few additional "$..." commands beyond the Grbl set, as described in FluidNC Commands_and_Settings. FluidNC implements very few of the Grbl $nnn (nnn is a number) settings. Those numbered settings are for machine configuration. FluidNC configures the machine via an hierarchical configuration scheme that is much more extensive than would be practical with numeric "names".
Anything you would do during a normal job is compatible with Grbl
FluidNC has many more features and options than Grbl. Grbl uses numeric $ settings. It was not practical to try to emulate or expand that system. For example: Grbl has one homing pull off value, where FluidNC has one for every motor. This adds a lot of flexibility, but breaks the setup compatibility.
We are planning on creating our own wizards. Our settings are text based and self describing. The WebUI has features for setting many of them.
Do not try to use Grbl setup wizards
Some senders require an exact startup message like Grbl 1.1f ['$' for help] You can set it to be anything you like with the $Start/Message command. See the details here.