Having GAD can be scary, especially when you combine it with other things. I too have GAD and depression and sometimes they sort of feed off each other.
Its hard to say exactly what you can do to help him since everyone is different, but you're certainly showing the right kind of attitude wanting to try.
The pain in his chest is most likely heart palpitations. The key thing to know about these is that although scary, they can't hurt you.
Understanding why it happens might be helpful, "why" in a physiological sense anyway.
Its basically the fight or flight response. The mind has perceived a threat and the body is preparing for you to either run away or stay and defend yourself. For most people, this occurs when something genuinely scary happens, but for people with GAD, this response can be triggered for what might seem like no reason. The reason may just be a thought, but it sets you off.
When this happens try and help your husband redirect his thoughts. Talk to him about something light, like something you are watching on TV, or how the weather is going. Something low pressure, but that engages his brain to think about something other than the initial thought that set him off. The two of you can make a decision that this is what you are going to do before anything like this happens, this way he knows you aren't ignoring his panic, simply trying to help him.
Another thing to try when he has the tightness in his chest is relaxation exercises...
Lay down or sit back in a comfy chair. Close your eyes and start at your toes, paying attention to each muscle group as you move slowly through them. Scrunch up your toes, then release... then move onto the calves, tightening and releasing. Move slowly up the body paying attention to each set of muscles. By the time you get to you head you should feel more relaxed.
Breathing exercises are also helpful...
Again, lay down or sit back and close your eyes, concentrate on your breath. Exhale all the air out of your lungs slowly through the mouth. Once you've exhaled completely, let the body naturally fill your lungs again. Just let it happen, don't force it. Feel the breath moving over your lips. Think about how it feels. Each time just letting the breath naturally fill your lungs.
It might be the case that your husband will find this works better if you say these instructions in a slow, calm voice for him as he lays down.
The exercise and trips out etc are great, keep doing that with him. If he needs some time to worry, let him do that. Set aside some time for him to feel upset, but limit it.
My psychologist taught me to save my worries for later by writing them down and allocating a window of time at the end of the day. Let him talk about his fears. They may seem silly, don't try and make him see reason, he likely knows that there is no real reason to worry, but he will worry non-the less.