Our Production Box running Windows Server 2016 does not have MS SDK tools for Global-Assembly cache (GAC)!!! So I copied the whole directory
C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools\
from my Development laptop (Windows 10 that has VS 2017 installed) to that production server where I want to have my .dll's registered to my production server's Global Assembly.

The assemblies I need to register are : _Net35scannersMonitor.dll which is x86 dotNet 2.0 and _Net40scannersMonitor.dll which is x64 dotNet 4.x.
At the prompt of directory where my copied gacUtil is, I entered:
gacutil -i D:\resource\Library\_Net35scannersMonitor.dll
and
gacutil -i D:\resource\Library\_Net40scannersMonitor.dll
I used the same gacUtil.exe (file version 4.6.1055.0) to do both. And they both end up at the right place dotnet2x86(C:\Windows\assembly) and dotnet4(C:\Windows\Microsoft.NET\assembly\GAC_MSIL) --
I don't have to use 2 different gacUtil.exe's.
Result:

Conclusion: Even your box has no VS installed, you can copy your gacutil.exe with its bin from the PC which has VS installed. No special download/install needed. Then you can use the copied gacutil for both x86 and x64 assemblies!!!