Hey Motorsep,
UE4 actually already supports the ASTC texture format. You can find it by going to File->Package Project -> Android then select the texture format you want from the list.
However when using UE4 to make projects for Gear VR you should use the ETC2 texture format. You can read more about Gear VR and UE4 development in the official documents that are linked below.
https://docs.unrealengine.com/latest/INT/Platforms/GearVR/index.html
Hope that helps and please let us know if you have any more questions.