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On-die memories, which are traditionally implemented by SRAM, stop functioning properly when the supply voltage is scaled down aggressively; hence, embedded DRAMs (eDRAMs) are used instead. Opportunistic refreshing was proved to eliminate the performance loss incurred by the eDRAM refreshing must. We show here that Gain-Cell eDRAM (GCeDRAM) supplemented with opportunistic refreshing consumes significantly smaller power and energy than SRAM. Analysis supported by hardware simulations demonstrate that the same design point achieves maximum performance and minimum energy. Replacement of the data memory in the ultra-low power processor PULPino from SRAM to opportunistically refreshed GCeDRAM yielded 30% energy savings in the memory, which translated into 7% savings in the entire processor.