Integrating navigation, drug delivery, and diagnostic sensing within a single hydrogel-based platform remains a key challenge in the development of multifunctional biomedical soft robotic systems because these functions impose conflicting material requirements that are difficult to satisfy within a monolithic construct. Here, we report a magnetically guided modular hydrogel robot assembled through sequential photocrosslinking of three independently functionalized modules for magnetic actuation, drug delivery, and surface-enhanced Raman scattering (SERS)-based biosensing. The modular architecture enabled stable mechanical integration while allowing independent optimization of each functional module. By combining mechanically robust polyethylene glycol diacrylate (PEGDA) with highly porous gelatin methacryloyl (GelMA), the platform achieved tunable drug release behaviors ranging from sustained to rapid release. The SERS biosensing module achieved sensitive detection of interleukin-6 (IL-6) with a limit of detection of 0.730 pg/mL, while open-loop magnetic actuation enabled effective three-dimensional navigation. As a proof of concept, the integrated robot was evaluated in a PDMS tubular gastrointestinal tract model, where it successfully performed magnetic positioning, localized drug release, and post-retrieval IL-6 detection. These results demonstrate the feasibility of a retrievable modular hydrogel robot that integrates navigation, therapy, and molecular diagnostics within a single platform, providing a versatile strategy for localized drug delivery and post-retrieval biomarker analysis.