Gomtuu was a sentient spaceborne organic creature that functioned as a space vessel that lived for several millennia up through the 24th century. It survived a near-extinction of its race and may have been the last of its kind. It seemed to have been "born" far from Federation space, possibly in another galaxy. Upon its discovery by the United Federation of Planets and the Romulan Star Empire, it was dubbed "Tin Man" and the "Star Creature", respectively.
Viewed from the exterior, Gomtuu was a massive, dark-colored starship shaped like a sunflower seed shell. Its interior cavity consisted of a series of chambers and corridors with gravity and atmosphere to support carbon-based lifeforms as a crew, with which Gomtuu lived symbiotically. Its amorphous interior was also capable of growing furniture and equipment from its walls and floors. It could project images from its exterior to the occupants.
Like a traditional starship, Gomtuu possessed a warp drive, transporter capability, and was fully equipped to defend itself with a protective force field. However, much of its technology was beyond that known to Federation science. It was telepathic over an extremely large distance.
Gomtuu was manned by a crew until an explosion in space generated radiation that penetrated Gomtuu's "hull" and killed the crew. Gomtuu wandered the galaxy for millennia after the traumatic experience, vainly searching for more of its kind. Alone and without a purpose, it eventually decided to commit suicide by orbiting Beta Stromgren, a star for which supernova was imminent.
In 2366, "Tin Man" was detected when Starfleet sent the Vega Nine probe to monitor the supernova. At the same time, the Romulans also discovered Gomtuu and a race was on to get to it first. As the alien did not respond to traditional communication methods, the Federation sent Tam Elbrun, a highly telepathic Betazoid, to make first contact.
The Federation established contact first through Elbrun, who telepathically warned Gomtuu of the Romulans. Gomtuu then demonstrated its defensive capabilities, destroying the Romulan Warbird and heavily damaging the USS Enterprise-D utilizing an energy wave. Later, with Elbrun and Data on board, Gomtuu came about and fired a second energy wave which propelled the Enterprise-D and a second Warbird 3.8 billion kilometers through space.
Gomtuu then teleported Data back onto the bridge of the Enterprise (through the Enterprise's shields) and disappeared with Elbrun to parts unknown. Elbrun provided Gomtuu with an alternative to suicide, and Gomtuu provided Elbrun with tranquility. (TNG: "Tin Man")
According to the script for the episode,  the pronunciation for Gomtuu was "GOM-too".
According to Michael Okuda, in the TNG Season 3 DVD special feature "Departmental Briefing Year Three - Creating the "Tin Man"", the design of Gomtuu was based on Richard D. James' vision of a vessel with a "very organic look." The studio model, built by Greg Jein, would be "partly based on [the look of] a peach pit." The Star Trek: The Next Generation Companion (2nd ed., p. 123) adds that "the model was another Rick Sternbach creation... It was designed in homage to the thermal pods in Buckaroo Banzai."
The interior was based on an idea proposed by Richard James, where the set was coated with spray foam, which gave the organic look he was looking for. According to the Star Trek: The Next Generation Companion (2nd ed., p. 123), "Rob Legato created the organic chair that seems to form out of Tin Man's very structure by reversing a time-lapse sequence of a melting wax chair."
The sounds from Tin Man's interior were a combination of whale sounds and a recording of sound designer Jim Wolvington digesting pizza, recorded through a stethoscope. The explosion (pictured above) was a scene reused from the final V'ger scene in Star Trek: The Motion Picture.