If I understood correctly, you need to read the Word document starting from your keyword to next heading. In other words, something like the red text in the following document:

In that case, here is how you can accomplish that with GemBox.Document:
string keyword = " [blah blah, blah, 001]";
DocumentModel document = DocumentModel.Load("input.docx");
ContentPosition start = document.Content
.Find(keyword)
.First()
.End;
ContentPosition end = new ContentRange(start, document.Content.End)
.GetChildElements(ElementType.Paragraph)
.Cast<Paragraph>()
.First(p => p.ParagraphFormat.Style != null && p.ParagraphFormat.Style.Name.Contains("heading"))
.Content
.Start;
string text = new ContentRange(start, end).ToString();
The text
variable's value will be:
Sample text content that we want to retrieve.
Another sample paragrap.
Also, here are additional Reading and Get Content examples, they contain some useful information.