Yes, GE Silicone 1 is safe for use in both fresh and saltwater aquariums and it does say not for aquarium use right on the tube. I've repaired several aquariums that housed fish for many months after with no issues. It is important to get version 1 and not 2 since version 2 has mold inhibitors that could be bad for fish.
Last time I bought a tube at the hardware store, it was about $5.00. The fish store equivalent for a smaller tube was $13.00. I've had my caulking guns for years so don't recall the cost, but I'm guessing they're between $10.00 - $20.00. They're one of those tools that last forever and can be used for household projects as well so are a good thing to have.
On the chance they ever change the product, always confirm the tube still says "100% Silicone" and there's no mention of new additives.