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I was experiencing the same issue and after trying many things (changing joint controller types, increasing mu values, decreasing slip, decreasing minDepth, modifying the physics properties of the block that my robot was grasping etc.), What ended up solving this problem for me was to specify the kp and kd values for the gripper fingers and to increase the kd value until the block stopped slipping out of the robot's grasp. Below is the example section from my robot's urdf file (using the LoCoBot Gazebo package):

<gazebo reference="finger_r">
    <mu1>1000</mu1>
    <mu2>1000</mu2>
    <kp>1000000.0</kp>
    <kd>100000.0</kd>
    <slip1>0</slip1>
    <slip2>0</slip2>
    <minDepth>0.001</minDepth>
    <maxVel>0</maxVel>
    <torsional>
        <ode>
            <coefficient>1000</coefficient>
            <slip>0</slip>
        </ode>
    </torsional>

</gazebo>

<gazebo reference="finger_l">
    <mu1>1000</mu1>
    <mu2>1000</mu2>
    <kp>1000000.0</kp>
    <kd>100000.0</kd>
    <slip1>0</slip1>
    <slip2>0</slip2>
    <minDepth>0.001</minDepth>
    <maxVel>0</maxVel>
    <torsional>
        <ode>
            <coefficient>1000</coefficient>
            <slip>0</slip>
        </ode>
    </torsional>
</gazebo>

You might have to experiment with these values a bit, but I hope this is helpful to anyone who is having this same issue!