Is the work done by the gravitational force on an object falling freely under gravity positive,negative,or zero?

$A$ man starts walking from a point on the surface of the Earth (assumed smooth) and reaches the diagonally opposite point. What is the work done by him?

$A$ particle of mass $m$ is projected at an angle $\alpha$ to the horizontal with initial velocity $u \ m/s$. What is the work done by gravity during the time it reaches its highest point?

$A$ block of mass $m$ is pulled along a circular arc by means of a constant horizontal force $F$ as shown. Work done by this force in pulling the block from $A$ to $B$ is ............

$A$ body moves a distance of $10 \, m$ along a straight line under the action of a force of $5 \, N$. If the work done is $25 \, J$,the angle which the force makes with the direction of motion of the body is.....$^o$

Explain work done by a constant force.