One way to do this:
1. Make a copy of the original spec (you'll have to name it differently)
2. Remove the old gasket in the copied spec and replace with new.
3. Change all the objects on the old spec to the new spec via the properties palette.
4. Change gasket in the old spec and move everything back if you want to retain the original spec name.
Note: I would recommend watching closely how your components are shifted to adjust to the new gasket thickness. It might be a good idea to start with just a few items and check the results.
I don't think there is any reason to select just the gaskets - unless I'm missing something. After creating the spec copy with the correct gasket, you will select all components with the old spec and change them to the new one. This will allow Plant 3D to insert the new gasket while adjusting the existing piping to fit. It MAY adjust if you change just the gaskets but I don't know for sure ...
If you need to select just the gaskets I may be able to come up with something.
I sent you a PM.