If you are preparing a deployment package I strongly advice against using regasm, use the installer class and the deployment project "Custom actions" instead (see Michael Knight response to your other posting regarding the installer class in C#). But if you still want to use it:
The utility is located in the same folder as the gacutil: e.g. C:\WINDOWS\Microsoft.NET\Framework\v1.1.4322 (if your Framework version is 1.2.4322); this folder is not added to your path so you will have to enter the whole path for the OS to find the executable.
The utility is distributed with the SDK, not the redistributable version of the Framework(one reason not to count on it ...).