If you are smart about it, you may be able to avoid a memory block. Depending on the properties of your numbers (for example, if 1+GC is invertible), you may be able to rewrite the equations:
dy/dt = A-B*y-C*(dx/dt)+D*x
dx/dt = E-F*x-G*(dy/dt)+H*y
dx/dt = E-F*x-G*(A-B*y-C*(dx/dt)+D*x)+H*y
dx/dt = (E-F*x-G*(A-B*y+D*x)+H*y)/(1+GC)
