For example:
Config.groovy:
// ... grails.variable1 = \"a\" grails.varibale2 = \"${grails.variable1}bc\" //...
UPDATE 1
You need to declare a variable:
def rootVar = 'a' grails.variable1 = rootVar grails.varibale2 = "${rootVar}bc"
Or you might be able to do it via a closure (not tested):
grails.variable1 = 'a' grails.varibale2 = { -> "${grails.variable1}bc" }()