Which firmware to use on printer settings for DIY printer
Unless you can find a slicer that explicitly supports grbl, you will need to check the G-code specification and see which common flavour is closest to what grbl supports. The RepRap Wiki includes grbl in it's compatibility chart, but it has question marks against a lot of entries for grbl.

The other alternative is to slice a very simple model and see what commands are issued by the slicer, and whether they are supported by grbl.

Note that there is a question as to whether grbl supports FDM printing technology, i.e. 3D printers that use filament. Most slicers are designed to work with FDM printers, and will expect to be controlling at least one extruder. If your printer does not have an extruder that is controlled via G-code, you may encounter real difficulties generating G-code for it using commonly-available slicers. 

RepRap Wiki: G-code