There is actually another metallic element that melts at a slightly lower temperature than gallium: caesium (28.5 °C). This fits the question in principle, and it's a bit unfair (IMHO) to rely on the actual melting point to disallow it as an answer – the difference being only 1.4 °C.
As far as I can make out (I found it surprisingly hard to verify this on the Internet) the metal with the next lowest melting point is rubidium (39.3 °C); and phosphorus (44.15 °C) may be the next lowest.
As caesium and rubidium are both radioactive, phosphorus is never found on Earth in its native form because it's highly reactive, and mercury is highly toxic, gallium is very suitable for use in thermal management applications. This may be why its low melting point caught the eye of the question setter, rather than than that of caesium.
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