How to check if an instance is of type String or GString in Groovy

Instanceof tests should work:

assert "fasd" instanceof String
assert "${this}" instanceof GString

Another way is to use the in keyword:

groovy:000> t = "hello"
===> hello
groovy:000> t in String
===> true
groovy:000> "${t}" in GString
===> true

The in keyword is the membership operator, and gets translated to an isCase call on the class:

groovy:000> String.isCase(t)
===> true
groovy:000> GString.isCase("${t}")
===> true

You can also use the .class method on a Groovy object

def a = "test"
def b = "${a}"

assert a.class == String
assert b.class == org.codehaus.groovy.runtime.GStringImpl

Note that a GString is easily evaluated and turned into a String, e.g. by calls to toString.

If you're looking for template-like functionality to have re-usable string definitions to pass around and leverage, you should consider the Groovy template engine.

see http://groovy.codehaus.org/Groovy+Templates