A GWR level gauge system for determining a filling level in a tank. The system comprises a tank, a transceiver, a surface waveguide comprising a single conductor transmission line probe, connected to the transceiver, arranged extending vertically into the tank and configured to guide the electromagnetic transmission signals towards the surface and to guide the reflected electromagnetic signals back to the transceiver; processing circuitry connected to the transceiver and configured to determine the filing level based on received reflected electromagnetic signals; a plurality of retaining elements arranged in fixed positions in relation to an inside of the tank and spaced apart along the probe, wherein each of the retaining elements circumscribes the probe, and is configured to allow movement of the probe in a vertical direction relative the retaining element, and to restrict movement of the probe in a horizontal direction relative the retaining element.