Ok, let me tackle this without aggression.
Please keep up with my explanation:
Opponent sends torkoal. Sun shines brightly.
Now you send gigalith, unleashing a sandstorm which disrupts sun.
Opponent then sends exeggutor. However, the weather is sand and therefore exeggutor lost his ability and therefore the chances of it sweeping are slim. Also, the main problem here is not having giga drain on exeggutor. Is the fact you assume gigalith stays on exeggutor, which it doesn´t. Gigalith did its job to counter sun already, so all it has to do is to switch safely to something that can now counter an exeggutor outside sun, which isn´t that hard.
tl;dr: After gigalith disrupts sun, no one stays with it against an exeggutor.