As far as i can see, the clearpath_husky stack does not provide any transforms out of the box. However you can get odometry messages from the husky_bringup package on the /encoder topic. These can be used as input for robot_pose_ekf or laser_scan_matcher, which both can publish the required odom(world) -> base_link transform.
In your case the laser_scan_matcher could even be used without any odometry data from the husky plattform at all. You just need to feed your laserscans into it, set ~fixed_frame to "odom" and both ~use_imu and ~use_odom to "false". Even if this odometry is far from optimal, it should give you a starting point for the gmapping usage.