You don't "gain" credit. Creditors view your credit profile, which includes a basic history of your credit-based accounts, sort of like your resume of credit - and decide whether or not to extend you a line of credit.
THey get a credit "score" which is a number based on a bajillion factors that do not make sense to humans as part of it.
By paying your bill and not carrying a balance - you are building your credit history by not having late payments or charge-offs appear instead of "pays as agreed". This - in and of itself is the way to get more and more credit - which may not always be the best thing.
The largest effect on your credit score is the length of time that the accounts have been open, followed by the amount of derogatory accounts shown.
As creditors see your current balance - if you carry one - less is better, but 0 is not the best. Creditors want to see you actually use the credit they extend, so the scoring model puts the largest positive impact that "balance" has - on an account with 5-10% of the maximum balance used.