GASKETS ARE NEVER DIMENSIONED IN ISODRAFT BUT HAS LEADER LINES. WHO OR WHAT CONTROLS THE GAP DISTANCE. CAN GASKETS BE DIMENSIONED? HOW?

Hello, 

Can you show a small screenshot of the leader line that you mentioned? 

I think that you meant dimension "extension lines" instead of "leader lines".

These extension lines are from the dimension of the flange and other component that are connected to the gasket.

Gaskets cannot be dimensioned because they do not have symbol key.
In our practice, we use a String-type UDA to store the gasket thickness, then we use OUTCOM to display the gasket thickness in the isometric drawing. It will show just as a text with the gasket thickness and a leader line pointing to the gasket in the isometric drawing, without a dimension line.

If you really want to display the dimension of a gasket with dimension line, then inside AVEVA Catalogue, you can create your gasket in another GTYP like PCOM. Then you can use a default SKEY or a user-defined SKEY. However, the disadvantage of this is that your gasket items will be mixed with other PCOM items.