How can a steroid user go off of his/her cycle without losing all their gains?

You won’t lose all your gains. You’ll lose some of them for sure, but as long as you keep working during your PCT you’re going to lose about a 3rd to 2 3rds of the gains from the cycle at the most, and it may seem like more, but you have to remember that most exogenous androgens cause edema, so you’ll be pretty wet during the cycle holding a bunch of water, so while it may seem like you get a lot flatter, much of that is water.
Another thing not oft discussed (and the reason why once you enhance yourself you’ll never be natty ever again, no matter how much time passes), is during a cycle the amount of androgen receptors actually increases permanently, especially in the receptor dense regions such as delts, traps and the upper portions of the pecs. You will often see these areas be “overdeveloped” in enhanced lifters and this is why. Once PCT is complete and folk return to their base test levels, they are still going to respond better to the anabolic properties of their natural testosterone than if they had never taken anything at all, and as I mentioned this is permanent.
There is also muscle memory too. The ability to regain lost muscle at approx 3x the speed as it first took to build it. And while this isn’t foolproof for super physiologically gained contractile tissue, it still does come into play at least a little bit.
hope that answers your question.

After you stop exercising, you will tend to lose all the extra muscles you gained due to the steroid usage.

Quoted Ziller33; Post# 3825

Kindly respond.

During a cycle of androgen use, you may lose some gains during post-cycle therapy, but it's usually only a third to two-thirds of what was gained. Androgen use can increase the number of androgen receptors permanently, particularly in specific regions such as delts, traps, and upper pecs, which leads to better natural testosterone anabolic responses after the cycle is over. Additionally, muscle memory allows for the possibility of regaining lost muscle at a faster rate than it took to build it initially.