gacutil is not recognized as an internal or external command?

If you already installed Visual studio then you can use Developer Command Prompt instead of normal Command prompt to access the gacutil directly.

You can find Developer Command Prompt here:

Start Menu > Visual Studio 20XX > Developer Command Prompt for VS20XX

probable duplicate : GAC 32bit vs. 64bit (gacutil is no more part of client OS since .Net 3.0 at least, you need to use a proper installer or to install the windows SDK which is clearly overkill)

"But how can i deploy my dll on Client Server because he might don't have visual studio"

If I remember correctly, gacutil is also present in C:\windows\Microsoft.Net\some folder\

Another way: If using a primitive .cmd => You could bundle gacutil.exe and gacutil.exe.config along with your script.

Yet another way: If you are using windows installer, that has in built support for GACing assemblies.

Also, note that there are different GACs for x86 and x64 assemblies (ignore if your assembly is MSIL).

For Windows 10 path is: C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin