  The issue of finite magnetic compressibility in low-beta magnetised plasmas
is considered within the gyrokinetic description. The gauge transformation
method of Littlejohn is used to obtain a Lagrangian which contains this effect
additionally. The field theory version obtains a system model which guarantees
exact energetic consistency. Gyrocenter drifts under this model are considered
within a Chew-Goldberger-Low MHD equilibrium allowing for pressure anisotropy.
The contributions to the current divergence balance, hence the dynamics, due to
the difference between the curvature and grad-B drifts and to the
compressibility are shown to cancel up to corrections of order beta. This
recovers an earlier result with the same conclusion within linear theory of
kinetic ballooning modes.
