On Frame view, you can check each frame of the video with a still image view and edit them.
Tap the thumbnailed image to deselect and re-tap to re-select. To check the full image, tap the magnifying glass icon.
Tap button to select some frames of the entire frames. Since the size of the GIF file gets affected by the number of frames, when you pick out some similar frames, you can make the size of GIF file smaller.