Seems like a bug with either selenium or geb or geckodriver. I reproduced the same with these dependencies:
dependencies {
testCompile 'org.gebish:geb-junit4:0.13.1'
testCompile 'junit:junit:4.8.1'
testCompile 'org.seleniumhq.selenium:selenium-firefox-driver:3.0.0-beta3'
}
Capabilities [{rotatable=false, raisesAccessibilityExceptions=false, appBuildId=20160623154057, version=, platform=XP, proxy={}, command_id=1, specificationLevel=0, acceptSslCerts=false, browserVersion=47.0.1, platformVersion=6.1, XULappId={ec8030f7-c20a-464f-9b0e-13a3a9e97384}, browserName=Firefox, takesScreenshot=true, takesElementScreenshot=true, platformName=Windows_NT, device=desktop}]