adb shell su -c command throws Permission Denied (My phone is rooted)

Solution 1:

A shell terminal does not throw any exceptions and your command line is wrong - the path, too:

adb shell "su -c 'echo 680000 > /sys/kernel/gpu/gpu_max_clock'"

You might end up with a defect GPU, when not having the slightest clue of what you're doing. Better research the max values first - when it goes up in smoke, this is your own fault, not mine.

Any sane person would probably read the original clock speed first:

cat /sys/kernel/gpu/gpu_max_clock

Only then one can see, how many percent of the original clock speed 680000 would even be. When giving to much, this will lead to overheating and random crashes... unless also reducing the voltage. This is by far not as easy as you may think it is ...and there is no active cooling available, either.