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A gyrokinetic linear stability analysis of a collisionless slab geometry inthe local approximation is presented. We focus on $k_\parallel=0$ universal (orentropy) modes driven by plasma gradients at small and large plasma $\beta$.These are small scale non-MHD instabilities with growth rates that typicallypeak near $k_\perp\rho_i\sim 1$ and vanish in the long wavelength $k_\perp\to0$ limit. This work also discusses a mode known as the Gradient Drift Coupling(GDC) instability previously reported in the gyrokinetic literature, which hasa finite growth rate $\gamma= \sqrt{\beta/[2(1+\beta)]} C_s/|L_p|$ with$C_s^2=p_0/\rho_0$ for $k_\perp\to 0$ and is universally unstable for$1/L_p\neq 0$. We show the GDC instability is a spurious, unphysical artifactthat erroneously arises due to the failure to respect the total equilibriumpressure balance $p_0+B_0^2/(8\pi)=\text{constant}$, which renders theassumption $B_0'=0$ inconsistent if $p_0'\neq 0$.

Gyrokinetic theory of slab universal modes and the non-existence of the Gradient Drift Coupling (GDC) instability