Then it is not a "gift". You have "done" something for salvation. Whether it is exercising "faith" or taking the Eucharist, you are adding to the work of God.
No, it is not us adding to the work of God. It is God working through us to add to His work. Exercising faith or taking the Eucharist is His helping us to respond to His call. It's our work only if we reject His call.
It is always a gift, because no one will die in a perfect state of theosis, so whatever sins you may have left will be forgiven by none other than God's grace. God is under no obligation or necessity (as your aside believes) to save you regardless just because you proclaimed "Lord, Lord" once in your life and are now "saved."
And YES we have to "DO" in order to be saved. We are responsible for our conduct and morality and virtue that God gave us. First and foremost, we have to repent. Not once, but over and over, because you sin over and over. We have to show works through faith. We have to become as Christ-like, as god-like as possible wihtin our life-time, as perfect as possible.
God doesn't expect us to succeed in becoming perfect, but if we fail we must HONESTLY fail by honestly TRYING to succeed. Yes sir, it's a struggle: first the cross, then the crown.
By obeying the commandment of Christ to celebrate the Eucharist I add to the work of Christ?
First of all, "doing a work" is not the same thing that Paul belittles in Romans and Galatians. You would be very hard pressed to tell me that Jesus didn't say we had to "do" something to enter the Kingdom. We are commanded to obey. It is the inner motivation, earning something, that sets apart Paul's notion of work and the notion of "doing" something.
Second of all, receiving the Eucharist is not adding to the work of Christ, it is RECEIVING the work of Christ...
Regards