How to copy/install assembly in a GAC? (Some problem running NUnit)

I've just had a weird problem on the build server. For one of the test projects, it would not build at all. The error message was the following:

C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1360,9): warning MSB3245: Could not resolve this reference. Could not locate the assembly "nunit.framework, Version=2.5.5.10112, Culture=neutral, PublicKeyToken=96d09a1eb7f44a77, processorArchitecture=MSIL". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors. 

Apparently, no NUnit library could be found. Checking the GAC manually, this appears to be true. How weird. Tried reinstalling and repairing NUnit 2.5.5 a few times with no result. Then, in Visual Studio Command Prompt, adding NUnit dll to GAC manually with

gacutil /i nunit.framework.dll

solved the issue.

Reference:

How to copy/install assembly in a GAC? (Some problem running NUnit) - Stack Overflow