For softies, gel will be preferred, and in fact it is not that great either.
I used to just attach them as close to the rock as possible with a piece of cooking string, then when the coral attaches itself to the rock, remove that.
If you want to attached a mushroom to a rock it's best to put the mushroom in a container with some rock rubble, eventually it will attach and then you can place it on the rock or sand bed in your tank.