2 ways I know of:

1. https://www.tenforums.com/tutorials/77458-rundll32-commands-list-windows-10-a.html

Use Dragon to run:

"Rundll32.exe shell32.dll,Control_RunDLL Mmsys.cpl,,1"

Above takes you straight to the classic Sound --> Recording tab.  From there use Dragon/KB to navigate up and down to select the microphone you want to control (in our evil days of the Corona and remote up-the-yin-yang most folks have several mics attached to their rig, as I do).  Use Dragon to double-click the Mic of your choice.  Click/Select "Levels".  Then use Dragon/KB to drive the left or right arrow to move the gain slider.  Not being a KB user, I am nevertheless 100% certain KB can automate that for you in style.  

2. Download AutoHotKey.  Run Soundcard Analysis script once (or every time you add/remove a mic), to find out the device number of the sound device you want to control.

Let's say the device number corresponding to the mic of your interest is 6, which you can easily find out by manually setting the mic gain to any odd number (say 39) and run the Soudcard Analysis script above to see which device number has "39" beside it.   

Then it is easy as pie (and if you find anything peasier let me know): 

SoundSet,100,,,6

You can bind above to a hotkey or call it with ShellExecutethisandthat from Dragon, which has been liberally mentioned umpteen times a day around here.

To "lock" the gain in place, you can run "SoundGet" every so often against the device number.  If it is not 100, set it to 100.

P.S., there must be 25 other ways to approach this on Windows, from PowerShell, VBS, etc. etc.  But simpler than AHK without installing something proprietary?  Likely bar none.

Non-pro tip from me: so long as you are tethered to this sorry platform called Windows: your choice of mitigation depends on where you wish to go:

Go sexy, go Python.

Go pro, go .NET (C#, VB, etc.)

Go home (as in wanting to accomplish what you need to get done and head home), get a shot of AutoHotKey.