Have a look at
print(.packages())
If "gdata" is inside, then the package is successfully loaded. I suspect the package has been loaded, because you got the warning message for masking.
You don't need to worry too much about masking. You can always do gdata::nobs and stats::nobs to call nobs function in different packages. Similarly, you can always do gdata::object.size and utils::object.size to get the right function.
