Several structural primitives are implemented but only reachable through the sed brace mini-language on the replacement side of a regex (e.g. {+=p} page break, {+=c} column break, {+=s} section break, the default-break horizontal rule, footnote insertion, and column count via section style). That path is undocumented in --help, requires a regex anchor, and is awkward to address by index or literal-text anchor the way the other inserters are.

These already have working request builders in the codebase (InsertSectionBreakRequest, CreateFootnoteRequest, UpdateSectionStyleRequest, and the bottom-border HR via InsertText + border). Promoting them to first-class commands with the standard --index / --at <text> / --at-end / --tab selectors (the same surface insert-page-break already has) would make them discoverable and consistent:

gog docs insert-footnote <docId> --at <text> --text <footnote body>
gog docs insert-section-break <docId> --at <text> [--type NEXT_PAGE|CONTINUOUS]
gog docs insert-hr <docId> --at <text>
gog docs section-columns <docId> --count N [--at <text>]   # UpdateSectionStyle column count

insert-page-break is the proven template — these would mirror its selector handling. This removes the regex-only requirement for structural editing and surfaces capability that already exists but is effectively hidden.

Ref: documents.batchUpdate InsertSectionBreakRequest, CreateFootnoteRequest, UpdateSectionStyleRequest.