Stored finds can't evaluate expressions. You have to enter either literal data or just the name of a variable--can't add anything beyond the variable name.
But since you already have the dates in global fields, you do not need the variables at all.
Do it this way:
Enter Find mode  -->no stored criteria, clear the pause check box.
Set Field [Visits::VistitDate ; Globals::gStartDate & "..." & Globals::gEndDate ]
For other examples of scripted finds that use global fields and set field script steps, see: Scripted Find Examples
Note: because you are using Show Custom Dialog, it's possible to enter invalid dates as well as dates where EndDate comes before StartDate. You may want to add some code that checks the data in the two fields before attempting to find records.
That did indeed do it.
I'll learn this thing yet.