You stated that you are not looking for developers. Maybe, you just want to gather the information for decision-making purposes.

I would say, it is possible to build a site that replicates the functionality of coingecko and/or coin market cap but the issue is this; you would have to do a lot of hype work in order for people to know that this site of yours exit.

This is so because there are lots of website like this which do not have the prominence of Coingecko or Coin Market Cap.

If you are a developer, then you need to get the blockchain APIs for all the major crypto currencies on major exchange platforms.
But if you are not a developer, you will need to learn some front end and back end skills first of all