Gravity feed is certainly type-specific, so there can be no generalizations.
On the 744, the only possible gravity feed between tanks is the automatic feed of the reserve tanks to wing tanks 2 & 3, and the selectable feed of tanks 1 & 4 into 2 & 3 (done in non-normal situations only).
"Gravity feed" of fuel from the wing tanks to the engines is possible in some cases because the engine-driven fuel pumps will supply the engines if all boost pumps fail. However, at high altitudes and power settings there may not be enough pressure head on the fuel tanks from the vent system to reliably move fuel down the pipes to the engines. That will cause cavitation of the engine-driven pumps and likely flameout. High pitch angles with low fuel quantity will also uncover the boost pump inlets, interrupting fuel supply.