Gallium (Ga) displays several metastable phases. Superconductivity is strongly enhanced in the metastable β-Ga with a critical temperature Tc=6.04(5)K, while stable α-Ga has a much lower Tc<1.2K. Here we use a membrane-based nanocalorimeter to initiate the transition from α-Ga to β-Ga on demand, as well as study the specific heat of the two phases on one and the same sample. The in situ transformation is initiated by bringing the temperature to about 10K above the melting temperature of α-Ga. After such treatment, the liquid supercools down to 232K, where β-Ga solidifies. We find that β-Ga is a strong-coupling type-I superconductor with Δ(0)/kBTc=2.00(5) and a Sommerfeld coefficient γn=1.53(4)mJ/molK2, 2.55 times higher than that in the α phase. The results allow a detailed comparison of fundamental thermodynamic properties between the two phases. © 2018 American Physical Society.