After Guiron cuts Gyaos' head off, the blood disappears from his head
In one of the film's most infamous editing gaffes, Gamera at one point appears in the water immediately after he's shown standing on land. It is not shown what caused him to fall in, it's as if a couple shots were missing.
When Guiron slices off Gyaos' left wing, Gyaos starts spinning clockwise. In the next shot, he's spinning counter-clockwise.
When the kids accidentally teleport themselves closer to Guiron, the teleportation device is to Guiron's right. In the wide shot, it's directly in front of him.
Dr. Shiga states that an Apollo spacecraft could get to Proxima Centauri in a million years. Proxima Centauri is a little more than 40 million million km away from earth, and the Apollo spacecraft could reach almost 40,000 kph; therefore it should only take about 100,000 years.
When Gyaos' beam is deflected back at him, his leg starts falling before it is cut off.
The seam on the Guiron suit is clearly visible.
One of the orange-brown stripes on Guiron's side starts peeling off during his fight with Gamera.
There are already ripples in the water before Guiron jumps into it to engage Gamera.
When the two kids launch a missile at Guiron, the missile gets sliced in half on his bladed head. But it is clear as the day that both "halves" are actually complete missiles. You can see the pyrotechnics go off behind Guiron's head that launch the second "half" of the missile.
When Guiron chops up Gyaos, he cuts him into clean pieces that seem to be made of the same solid, purple material, with no bones, organs or any sort of innards. However it has been previously shown that Gyaos' species has, among others, not even one but two spines.
Established by Gamera vs. Gyaos (1967), direct sunlight causes a color change to Gaos' head and would be lethal to the creature. Given that the planet Terra travels the same orbit as Earth around the sun, the same weakness should have been present in this film as it battles Guiron (when the children first arrive at Terra).
We would know if there was a 'counter-Earth' (a planet orbiting directly opposite us and hidden from view by the sun) due to its gravitational effects on the motion of the other planets.
(In the English dub) The two kids refer to planet Terra as a star.