Yeah.. i use GE Silicone I as well. It is aquarium safe. The reason it says on the tube, "not for use below the water line or aquariums," is because a while back, GE was working in compliance with AGA tanks, and another big aquarium manufacturer. All of the larger tanks 100G+ that used the GE silicone were beginning to fail, and split under pressure of the water. They no longer have that partnership with AGA or any other aquarium company, and in order to protect themselves, they say not to use it on aquariums.
In light of this, i wouldn't recommend using it to reseal a tank.. if it's a large tank, but if you want to use it to reseal one glass pane, or to secure baffles in a sump, i say go for it. I used it on baffles in my sump and this stuff is very strong and durable. You just need to let it cure for the amount of time specified on the bottle.
Also, just a little word of advice to all that may consider using GE I Silicone, if after a while, you start to notice the color of the silicone turn a yellowish color, do NOT worry. It happens to some people that use it, and sometimes it doesn't. It happened to me, but the color fades out in a few weeks to the normal clear color. However, the GE I silicone WILL turn a greenish/blueish color if there is copper present in your water... so beware if you have inverts or a reef tank and you see your silicone turn that color.
