Before you start asking questions, you need also to read the Boost guides, familiarise yourself with documentation and wiki pages of Boost and Boost.GIL specifically,
the GitHub space, etc.
Before you start asking questions about your project, there is a lot of homework you should do yourself,
nothing to guide you about, as everything can be found online, just do web search
@Cypre55 Web searceh for "how to update pull request" should give decent explanation
DO address PR feedback in an additional commit(s) rather than amending the existing commits. This makes it easier for reviewers to track changes.
DO sync your PR branch with the upstream develop branch frequently resolving any conflicts if necessary. You can either git merge upstream/develop or git rebase upstream/develop with git push --force for the latter. The merge may make it easier for reviewers to track changes though.