This value should be in the range [-16, 8] which equates to the range [0.8V, 1.4V] with 0.025V steps. In other words, specifying -16 will give 0.8V as the GPU/core voltage, and specifying 8 will give 1.4V. Values above 6 are only allowed when force_turbo is specified: this sets the warranty bit if over_voltage is also set.
gpu_freq This sets the core_freq, h264_freq, isp_freq, v3d_freq and hevc_freq together.
On the Pi 4 it is recommended to use the individual frequency settings (isp_freq, v3d_freq etc) rather than gpu_freq because changing core_freq is not supported, any change from the default will almost certainly cause a failure to boot, it is not likely to have the desired effect.
The following variables are individual overrides of the gpu_freq setting:
core_freq Frequency of the GPU processor core in MHz.
This influences CPU performance because it drives the L2 cache and memory bus; the L2 cache benefits only Pi Zero/Pi Zero W/ Pi 1, there is a small benefit for SDRAM on Pi 2/Pi 3.
h264_freq Frequency of the hardware video block in MHz
isp_freq Frequency of the image sensor pipeline block in MHz
v3d_freq Frequency of the 3D block in MHz
hevc_freq Frequency of the High Efficiency Video Codec block in MHz. Pi 4 only.