Captain Swan is credited for being an attacker because he possibly wrote a song (someone did, and it appears to have been him.)

Some persons list Thomas Swan of Bristol, as a participant.   Because he was from Bristol, we assume he was in the Bristol boat, but that is only an assumption.  There is a Thomas Swan born about 1749, died 30 Jul 1805, buried in Bristol, RI, who would have been about 23 years old at the time of the attack.

The Bristol boat was under the general captaincy of  Simeon Potter.  Potter's wealth and position were based on his pirate voyages that began when in sailed out of Newport in 1744 in command of a Newport-registered sloop, with a privateer's commission signed by Governor William Greene of Rhode Island.  By 1772 he was comfortably ashore,  but he continued to finance ships and build a considerable fortune.

It makes sense that if he was going to bring a boat of men to the attack,  Potter would have brought men from Bristol, and would have sought strong young men for the long row to be followed by force of a good fight.

The evidence for Thomas Swan, later known as Captain Swan,  having been a raider rests almost solely on the well-known "Gaspee Song", a Revolutionary War era poem immortalizing the event.  Swan has been the person most credited with writing this poem. E.g., Munro, writing in his 1860 The History of Bristol, R.I.- The Story of Mount Hope Lands, names Swan as the author. Swan had a handwritten copy of the poem,  and this is the evidence for him to have written the poem.  On the other hand, it could be surmised that Swan heard the poem, wrote it down in his own handwriting, and enjoyed reciting it at family gatherings.  

The children of Captain Swan had an oral tradition that Captain Thomas Swan wrote the poem himself. This claim was probably passed onto Munro when he did his historical research some 88 years after the burning of the Gaspee. This supports the Swan family claim of authorship, and we have found that strong family oral history existing in the first and second generation usually has great truth strength.

When the poem was printed (after the Revolution) the name of Theodore Foster was hand written on the title page, as though Foster was the author, of  the only known existing printed version of the poem.  From this it could be deduced that Theodore Foster was the author.

Foster was an ardent patriot, and of the right age to have participated himself in the Gaspee raid.  However the evidence is that, long after the Revolutionary War had ended, it was Foster who asked Mawney to write an account of the Gaspee affair for the benefit of history. (See page on Capt. Joseph Tillinghast.) It would be unlikely for Foster, a lawyer and prolific writer, to have asked another person to write down a recollection of the events and not do so himself.  Hence we come to the conclusion that Foster was not a Gaspee raider, even though Foster may have been the author of the poem, which is usually credited to Swan.