So first of all, the concept of "double taxation" is something you have to be careful with. In general, a given dollar will be taxed more than once because it is spent more than once. However, that same dollar also counts towards the net income of the country more than once. Double taxation is when the same income is taxed more than once, which is a bit different.

Capital gains in general does not involve double taxation. Some examples:

If you buy property for $100,000, and sell it for $150,000, you'll pay capital gains taxes on the $50,000. Here, $200,000 of income is involved and $200,000 is taxed.

Say you invest $100k in a store, and build it up to $100k/year in revenue. You sell it for $1m. The business has no assets to speak of (you lease the space, etc). You're taxed on the $900k. Here $1.9m of income is involved and $1.9m of income is taxed.

Where you do run into double taxation is with corporations. $1m of corporate income is taxed once when it is earned and again when it is distributed as a dividend. But that's because the corporate income tax is double taxation, not because capital gains is double taxation.

Also, there is no double taxation when paying salaries. Say you have $1m in revenue and $400k in expenses (salaries and rent). You're taxed on the $600k of net income, not the $1m in total revenue.