I think its okay to restrain a child if they are a danger to themselves or others. I don't think you should feel guilty. But I imagine its very difficult to restrain a 45-lb out-of-control child! I think putting your younger DD behind the baby gate is perfect to keep her safe.
You say you are trying to be proactive in heading off tantrums - do you want any more advice along those lines? If so, it might be useful to give us some specific examples so they we can help brainstorm ideas. If you feel you have a good handle on this, then nevermind.
Is there a room, maybe his bedroom, that you can tantrum-proof? Move all hard toys to another place, strap down or nail down chairs or furniture that can be thrown, etc. Make it a safe place so that when he is in there, he does not have to be restrained because there is nothing that can hurt him (or you).
In this place, add some "comfort" things:" Maybe a couple bean bags with blankets that you two could cuddle together on, a CD player (up high on a shelf or something), etc.
Also have in this place some good activities to honor his need to be physical when he is upset. You could have a pile of nerf balls that together you can fling at the wall or each other when he is upset, for example.
When he becomes upset, you can ask him to come with you to his "calm" spot. If he doesn't come, you can, as gently as possible, escort him there. Go inside with him and close the door. Then you can do what you feel is best for him, either just wait it out, join with him, modelling the footstomping dance or flinging nerf balls while saying, "You are SO MAD right now!", reflecting his feelings, whatever seems to help. If he attacks you, I would deflect and redirect (but this is just me. You could step outside the door for a few seconds if you fear getting hurt). You could try different types of music (sometimes soothing music makes things worse and a child needs something with an aggressive beat which paradoxically helps them calm down; I don't have any idea why this is so). Eventually the idea is for him to use these other ways of handling his anger with little prompting from you; maybe someday he will be able to take himself to his room and do what he needs to do in there (throw balls, scream, play music) then come out when he is calm.