fix wrong message

src/com/sap/piper/ConfigurationHelper.groovy

@@ -184,7 +184,7 @@ class ConfigurationHelper implements Serializable {
 
     ConfigurationHelper withPropertyInValues(String key, Set values){
         withMandatoryProperty(key)
-        def value = config[key]
+        def value = config[key] instanceof GString ? config[key].toString() : config[key]
         if(! (value in values) ) {
             throw new IllegalArgumentException("Invalid ${key} = '${value}'. Valid '${key}' values are: ${values}.")
         }

test/groovy/NeoDeployTest.groovy

@@ -515,4 +515,17 @@ class NeoDeployTest extends BasePiperTest {
             utils: utils,
         )
     }
+
+    @Test
+    void deployModeAsGStringTest() {
+
+        Map deployProps = [deployMode: 'warPropertiesFile']
+
+        stepRule.step.neoDeploy(script: nullScript,
+                  utils: utils,
+                  neo: [credentialsId: 'myCredentialsId',
+                        propertiesFile: warPropertiesFileName],
+                  deployMode: "$deployProps.deployMode",
+                  source: archiveName)
+    }
 }