It looks to me you complicate things too much.
Here's a simple example that should do the job:
For given test.properties
file:
a=1
b=2
This code runs fine:
Properties properties = new Properties()
File propertiesFile = new File('test.properties')
propertiesFile.withInputStream {
properties.load(it)
}
def runtimeString = 'a'
assert properties."$runtimeString" == '1'
assert properties.b == '2'