No peer-reviewed scientific study has ever concluded this; many have concluded that the Gateway Theory is clearly untrue, and some have even concluded that marijuana use helps prevent the use of other drugs. ref1ref2ref3ref4
The explanation for the perceived gateway effect is that marijuana's prevalance leads to it almost always being the first drug encountered. Alcohol and tobacco are equivalant in percentage of hard drug users who first tried something else. Similarly, most motorbikers started out riding a normal bike. But it's clearly not true that riding a bike automatically leads you to ride a motorbike.