Go for BLF19 batteries. They are a great choice to keep on you to power your camera, particularly when you’re shooting video which can use up a lot of battery power.
A battery grip gives you an extra boost of power, providing you with a solid shooting platform that feels great in the hand. Try the Panasonic Battery Grip for GH5.
If you’re shooting video, you probably want high quality audio to go with it. Use the Panasonic XLR Audio Adapter for GH5, which allows you to use phantom power microphones and adjust audio settings on the go.