Geostrophic wind flows along straight isobars. As it is flowing in a straight line, the forces must be balanced. There are only two forces here (as you are above the friction layer): pressure gradient force and Coriolis.
Gradient wind flows along curved isobars. As it is flowing in a circle (or a curve) this is not a straight line and so the forces on it cannot be balanced!
Surface wind flows slightly across isobars towards the low. As it is flowing in a straight line, the forces must be balanced. As it is within the friction later, there are three forces here, pressure gradient force, friction and Coriolis.
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Which forces are balanced in the case of geostrophic wind?