I love this stuff! The only thing a teaspoon is good for is repeatability. Beyond that, a brewer has no idea what they are adding to their water. This is not to say that you can't brew that way and be successful. If you like what a teaspoon of gypsum in 5 gallons of water does for your pale ales, then that is good enough. You just won't know what that addition is actually producing in your water.
There is a reason why Bru'n Water includes only mass measures, it is the only way that a brewer is going to have any idea of the ionic concentrations in their water. When it comes to any mineral additions that alter RA, you really should use a scale.
PS: Scales are cheap.