The 14th century fornaldarsaga, Norna-Gestr þáttur, tells of its pagan protagonist Gest playing a harp in the Christian Olaf Tryggvason's hall. On that occasion he is said to have played a piece called Gunnarsslagr ('Gunnar's [harp]-striking'), a name indicating a dance tune.  That poem is lost.

Tekr Gestr hörpu sína ok slær vel ok lengi um kveldit, svá at öllum þykkir unað í á at heyra, ok slær þó Gunnarsslag bezt.

"Gest took his harp and played it well, and long into the evening, so that everyone was delighted to hear it, and he played the Gunnarsslag best."

On a trip to Iceland in 1780, Gudmundur Magnusson discovered a poem by that name in a paper manuscript along with other eddic poems. It certainly raised a few suspicions that the poem was young, but much more suggested to him that the poem was authentic ("authentiam") and he considered it equal to Vegtamskviða and Hrafnagaldur Óðins. Since this was also the opinion of other experts who examined it, Gudmundur Magnusson decided to print Gunnarsslagr as the thirteenth poem in the second volume of his Edda. In the first quarter of the 19th century, some considered the poem a heathen composition, despite the fact that its author made no attempt to present it as genuine.

In manuscript NKS 1877 4to, the poem is presented with these words on the cover page:  

Gunnars-lagr / Ein af þeim töpuðu kviðum / Sæmundar-Eddu, / fundin í Íslandi ár 1787 / af G. Magnæus / með fáeinum Leiðrettingum og útskýringum  

'Gunnars-lagr / one of the lost poems of Sæmund's Edda, / discovered in Iceland year 1787 by G. Magnæus / with a few corrections and explanations. "      

The poem was printed in the first edition of the Eddic poems, Edda Sæmundar hinns fróða, in Copenhagen 1787-1828, with a note in the introduction indicating that it was probably composed by clergyman Gunnar Pálsson. In 1818, August Afzelius published a Swedish translation of the poem in Sæmund den vises Edda.  In subsequent years, Peter Erasmus Müller (1818), Finnur Magnússon (1822), P. A. Munch (1847) and Niels Matthias Petersen (1865) all argued that the poem was young. As late as 1837, Ernst M. L. Ettmüller (1802-1877) defended the poem as genuine. Seemingly aware of the truth, Benjamin Thorpe included it in his mid-19th century translation of the Poetic Edda, with this remark:

"[T]he one entitled Gunnar's Melody is no doubt a comparatively late composition; yet being written in the true ancient spirit of the North is well deserving of a place among the Eddaic poems."

 Since then, there has been little interest in the poem.

Gunnar Pálsson (1714-1791) is known to have composed Gunnarsslagr, a poem in the style of the Poetic Edda concerning the death of Gjúki's son, Gunnar, as he plays his harp in a snake-pit facing certain death.  The reference in Norna-gests þáttur may have given him the inspiration. Gunnar Pálsson made no claim that Gunnarsslagr was ancient, nor made any effort to conceal his authorship of it. In a collection of his poems, gathered after 1777 and now preserved in the British Museum,  Gunnar refers to Gunnarsslagr as a 'new composition' that he sent to Eyjólur Jónsson. In a letter dated 23 Sept 1745, recently discovered by Haukur Thorgeirsson (2008), Eyjólur responded to Gunnar Pálsson regarding the poem. See "Gunnarsslagur and Valagaldur Kráku: Eddic poems from the 18th century."
The poem has an archaicized language and vocabulary and is clearly an effort to imitate a genuine medieval poem.  . It is usually regarded as a beautiful, well-constructed poem, quite unlike other eddic poems which commonly present textual and interpretative problems. The Thorpe translation (below) is correct in the main and mostly accurate, as the poem is easy to understand and free of interpretation problems.

 Both the poem and the translation are in the public domain.
The following edition is after that of Haukur Thorgeirsson.
In his translation, Thorpe appears to miss two words in the text;
the translation of which been inserted by Haukur in square brackets.

A Manuscript of the Poem can be found HERE.