There is a port of GCC available. On my distribution, it's available as avr-gcc.
Compile using that and (statically) link it into an ELF file.
Then run objcopy over it to obtain an IHEX version. You'll need to select (at least) the text and data sections. The result can be flashed onto the board via some port (/dev/TTYACMx, usually) using a program named "avrdude".