Avalanche fluctuations set a limit to the energy and position resolutions that can be reached by gaseous detectors. This paper presents a method based on a laser test bench to measure the absolute gain and the relative gain variance of a Micro Pattern Gaseous Detector from its single electron response. A Micromegas defector was operated with three binary gas mixtures, composed of 5% isobuLane as a quencher, with argon, neon or helium, at atmospheric pressure. The anode signals were read our by low noise, high gain CremaL CR-110 charge preamplifiers to enable single electron defection down to gain of 5 x 10(3) for the first Lime. The argon mixture shows the lowest gain at a given amplification field together with the lowest breakdown limit, which is at a gain of 2 x 10(4) an order of magnitude lower than that of neon or helium. For each gas, the relative gain variance f is almost unchanged in the range of amplification field studied, It was found that f is twice higher (f similar to 0.6) in argon than in the two other mixtures. This hierarchy of gain and relative gain variance agrees with predictions of analytic models, based on gas ionisation yields, and a Monte-Carlo model included in the simulation software Magboltz version 10.1. (C) 2014 Elsevier B.V. All rights reserved.