Since the DVFS firmware release, and its recent incorporation in the the release firmware, the gpu_freq option to overclock the GPU has not been working. We now have some test firmware which will, hopefully, reintroduce the ability to overclock parts of the GPU.
Note this does not overclock the core, but will attempt to overclock other blocks in the GPU to get them as close the the set gpu_freq as possible. Note that there is no guarantee any particular overclock will work, but this firmware may even allow faster overclocks than before (for example the 3D block has been clocked up to 800MHz in testing, although as mentioned, there is no guarantee you can hit any specific frequency as that is dependant on the silicon). You can still use the individual clocking options if you wish, for example, to clock blocks at different speeds if their maximum reliable speeds differ.
See this github issue for more details.
https://github.com/raspberrypi/firmware ... -581892520