Reasons for low melting point of gallium
For your interest, I quote a paragraph from a book "Handbook of Nanophysics: Nanoparticles and Quantum Dots" by Klaus D. Sattler (Ed.). Section 12-8. I don't think people know the reason as to why Ga likes to exist as a dimer (as you know $\ce{S}$ is $\ce{S8}$ and so on).


  The cohesive energy of the elemental solids is defined as the
  difference between the energy per atom of the solid and the energy of
  an individual atom in isolation. However gallium is rather unique
  among metals in that in the solid state it has a very unusual
  structure—an orthorhombic 8-atom unit cell composed of dimers which
  have a bond distance 15% shorter than the next nearest neighbor
  distance. As such, it has been called a “molecular metal,” which
  really contains two distinct kinds of bonding: covalent bonds within
  the dimers which result in a pseudogap at the Fermi level, and
  metallic bonding formed by overlap of the electronic wavefunctions
  along the buckled planes perpendicular to the molecular bond of the
  dimer. As a consequence, conduction in gallium is highly anisotropic,
  with a much greater conductivity along the buckled planes than
  perpendicular to this direction (Gong et al., 1991; Heine, 1968).
  
  If we estimate the “metallic” cohesive energy of the solid, by
  calculating the cohesive energy relative to the dimer, we immediately
  see that gallium moves much closer to the other elements, as shown in
  Figure 12.6. In fact, in this case we are still overestimating the
  cohesive energy relative to the melting temperature, since the dimers
  in the bulk will be distorted from the free dimer geometry. This
  argument certainly explains the low melting point of gallium relative
  to other metals, and due to the anisotropy of the lattice most likely
  relates to the surface superheating of gallium that was observed very
  early on (Volmer and Schmidt, 1937).