Just when you write software and publish it, you license it with GPLv3. If you use Github it will actually be one of the options to select a license for your code. As a user, source code is source code.

Personally, when searching for software, you can use its FOSS or F-droid for initial discovery, then search the name of the program in a search engine using the following syntax:

site:github.com obtainium

as an example this will tell the search engine you want results from github.com for the program called obtainium

OR

site:gitlab.com program_name

Again as a user, source code is source code, but if you want to prefer GPLv3 software (more popular software does tend to get more developers working on it afterall) you can then look at what license the program uses.