I had this same problem with my own GL753ve. I went so far as to reinstall Windows 10 along with all the drivers and this did not fix the problem. Within a minute of turning on the machine and without any apps open at all the fan would rev to high and stay there. Monitoring software always showed that the cpu and graphics processor were running nominally cool except right after startup, so they got hot enough to trigger the fans but cooled right down and stayed cool as long as the fans were running. Interestingly, the temperature monitors in the ASUS Gaming Center app reported zero degrees for the CPU and some crazy high thousands of degrees for the graphics processor even while the fans were running.

I finally decided to check under the hood. There was no dust at all and no other fan obstruction and the fan was spinning freely.

It turned out that the thermal paste between the heat sink and both the cpu and the graphics processor had creeped off and puddled around the processors on the motherboard. Using paper towels I cleaned off the paste from the heat sink and motherboard and wiped clean the paste from around the processors as well as the paste from the top of three other smaller chips (which had not slumped off the chips) that are also served by the same heat sink. I then reapplied a small bit of new thermal paste on each processor and chip and replaced the heat sink. This solved the problem. There is no longer a temperature spike on startup and even the ASUS Gaming Center is reporting nominal temperatures of about 35 degrees C.

I should note that this laptop was purchased new in September, 2018 (it is now July of 2020) and is only used occasionally mostly for email, web browsing and watching Netflix videos when traveling, about three or four days per month. It has never been used for gaming or other processing intensive activity. It seems that this creeping thermal paste is a design problem that will require periodic maintenance regardless of usage.

Disassembling the laptop is a fairly straight-forward matter, you can find videos that show the process on Youtube. You only need to remove the base cover then the fan/heat pipe/heat sink assembly.

The thermal paste I used was Noctua NT-H1, about $8 from Amazon for a lifetime supply. The amount to apply to each processor is illustrated in a diagram that comes with the paste. The smaller chips had much more paste on them than the processors which I matched when I did reapplied the paste. It looks like the chips sit further under the heat sink than the processors so the extra paste is needed to make up the difference.