Magnetic-field-induced semimetal-insulator phase transition in graphite has regained attention, although its mechanism is not fully understood. Recently, a study performed under the pulsed magnetic field discovered that this phase transition depends on thickness even in a relatively thick system of the order of 100 nm and suggested that the electronic state in the insulating phase has an order along the stacking direction. Here we report the thickness dependence observed under dc magnetic fields, which nicely reproduces the previous results obtained under the pulsed magnetic field. In order to look into the critical condition to control the phase transition, the effect of electrostatic gating is also studied in a field-effect transistor structure since it will introduce a spatial modulation along the stacking direction. Magnetoresistance, measured up to 35 T, is prominently enhanced by the gate voltage in spite of the fact that the underlying electronic state is not largely changed owing to the charge-screening effect. On the other hand, the critical magnetic field of the semimetal-insulator transition is found to be insensitive to gate voltages, whereas its thickness dependence is fairly confirmed. By applying positive gate voltages, a prominent oscillation pattern, periodic in magnetic field, becomes apparent, the origin of which is not clear at this stage. Although electrostatic control of the phase transition is not realized in this study, the findings of gate-voltage tunability will help determine the electronic state in the quantum limit in graphite.