Being given something is the same as gaining something. To gain just means to get (regardless of the means). If you give me 5 dollars then I just gained $5. You are mixing up the word gained with the word earned. When it comes to battle, whether you earned it or not is irrelevant.
You still don't get it, do you. I am simply trying to explain to you why people want Naruto and Sasuke to lose their powers (which is the topic at hand), not why Naruto and Sasuke's new powers [don't] belong to them.
When you gain something due to being given it, it lessens the impact of your achievement and makes you look reliant on assistance. I associated "gain" with "earn" because the former is usually associated with hard work or experience, when excluding current Naruto and Sasuke, rather than receiving; e.g., "Naruto
gained Sage Mode after his hard work and training at Myōbokuzan, as did Jiraiya", "Rock Lee
gained vast taijutsu experience and prowess due to his hard work and determination", "Hashirama and Madara
gained combat experience due to their many battles as children", "Orochimaru
gained a vast understanding of biology, history, jutsu after many years of studying".