How do I resize the partition for Android OS?

I just got my Rock5B recently. And what I want is use this device as a high performance Android TV Box.
I downloaded this image and flash to SD card. Everything works fine except Google Play.

I use dd to clone my Android OS from SD card to eMMC, the usable storage is still 8GB. How can I use the rest of space? I mean expand the last partiton to the end of this eMMC storage.

I tried to mount this eMMC to Ubuntu Desktop, and try to use gparted to resize it. But it seems these partitons is not formated as EXT3/4 file system. It just shows it is a Linux partiton. But don’t know what file system is. And don’t know how to expand/resize it.

Thanks for anyone’s help!