  We analyze the phase diagram of single and bi-layer cuprates using the
gauge-field description of the t-J model. For $T>T_{\text{BE}}$ the in-plane
fermion-pairing order parameter $\Delta_{\scriptscriptstyle\parallel}$ is
eliminated by gauge field fluctuations, leading us to predict the absence of a
spin-gap phase in single-layer cuprates. For bi-layer cuprates the inter-layer
order parameter $\Delta_\perp$ is enhanced by spin correlations, and is less
affected by gauge fluctuations. We believe that the spin gap in bi-layer
materials is due to inter-layer fermion pairing, and that for these underdoped
samples the superconducting gap may be nodeless.
