When Greg breaks his leg protecting Steven in "Ocean Gem," Steven allows his dad move into the Beach House until his leg heals and his van can be repaired. The Crystal Gems and Steven prepare to depart for a mission in a different dimension. Before they leave Pearl gives Greg the whistle. She tells him to blow the whistle if an emergency occurs, which would alert them to quickly return home. However, Greg uses the whistle repeatedly, causing Steven to rush back believing his dad is in danger. Steven scolds his dad, and Greg confesses that his leg had been healed the entire time. To make amends for lying, Greg uses the whistle to send himself via warp pad to the location of a cracked Geode and attempts to fix it.
It is likely a reference to the hidden warp whistles in the video game Super Mario Bros. 3 and/or the recorder from The Legend of Zelda.
Even though humans haven't been shown to travel or use Warp Pads alone, they can use the whistle to travel through or activate them, as Greg did to warp to and repair the Geode.