We study finite-blocklength lossy transmission of a Gaussian memoryless source over memoryless, possibly nonstationary block erasure channels under an excess mean-squared distortion criterion. We derive computable nonasymptotic achievability and converse bounds and establish matching third-order, channel-uniform normal approximations. Our nonasymptotic achievability bound follows from an exact evaluation of the ensemble-average excess-distortion probability of a specified random coding scheme and improves known general one-shot bounds (Kostina-Verdú 2013, Li-Anantharam 2021) evaluated with the same channel input and source reproduction distributions. Our nonasymptotic converse argument conditions on the erasure pattern and combines an energy-conditioned spherical-cap bound with a volume bound. The spherical-cap argument retains the geometric prefactor needed to identify the converse third-order term. In our channel-uniform normal approximation, we show that for fixed distortion ratio, target excess-distortion probability, and block size, the required source blocklength and remainder constants are independent of the channel blocklength and erasure-probability profile. Both the sufficient and necessary information-balance conditions contain the term $\frac12\log k$, where $k$ is the source blocklength, and differ only in bounded remainder terms. The analysis combines a threshold-uniform normal approximation for functions of the source energy, logistic and exponential random-threshold representations, and source-induced Gaussian smoothing of the channel information. Numerical evaluations compare the nonasymptotic bounds with their common third-order normal approximation. Our results provide a benchmark for transmitting a high-dimensional source using a small number of packets.

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