GCN Circulars distribute crucial information on astronomical observations of transient phenomena, but their flexible prose mixes observations, limits, metadata, contacts, and interpretation. We present an auditable pipeline that converts these reports into event-linked, structured records. Regular expressions provide domain hints; a large language model returns strict schema-constrained JSON; deterministic stages normalize values and emit review flags. Seven domain reviewers assessed 231 outputs from 210 distinct Circulars, fully confirming 223 reports and marking eight partly correct. They confirmed 25,827 of 25,880 definite field decisions (99.80%); errors concentrated in timing and facility attribution. Comparison with independent redshift datasets found semantic catalog disagreements but no values misread by our pipeline. Applied to the GCN Circular archive back to 2016, the pipeline has structured 26,811 reports into 68,393 observations, now served in real time by the Astro-COLIBRI platform. We release the extraction pipeline as an open-source package and the derived records as a continuously updated resource.