Jacobo Fitz-James Stuart, 6th Duke of Liria and Jérica
" Don" Jacobo José María Fitzjames- Stuart y Silva, 6th Duke of Berwick, 6th Duke of Liria, 6th Duke of Jérica, Grandee of Spain( 3 January 1792 – 5 January 1795) was the second surviving son of the 5th Duke of Berwick, briefly inheriting his titles.
He died at age three and two days and passed them onto his younger brother Carlos Miguel Fitz- James Stuart.
George Maximilianovich, 6th Duke of Leuchtenberg
Prince George Maximilianovich Romanowsky, 6th Duke of Leuchtenberg (29 February 1852 – 16 May 1912), also known as Prince Georgii Romanovsky or Georges de Beauharnais, was the youngest son of Maximilian de Beauharnais, 3rd Duke of Leuchtenberg and his wife, Grand Duchess Maria Nikolaevna of Russia.
Auguste, Duke of Leuchtenberg
Auguste Charles Eugène Napoléon, Duke of Leuchtenberg( 9 December 1810 – 28 March 1835) was the first prince consort of Maria II of Portugal.
Besides being the 2nd Duke of Leuchtenberg and 2nd Prince of Eichstätt, he also held the Brazilian noble title of Duke of Santa Cruz.
Charles Gordon-Lennox, 6th Duke of Richmond
Charles Henry Gordon- Lennox, 6th Duke of Richmond, 6th Duke of Lennox, and 1st Duke of Gordon,( 27 February 181827 September 1903), styled Lord Settrington until 1819 and Earl of March between 1819 and 1860, was a British Conservative politician.
Sergei Georgievich, 8th Duke of Leuchtenberg
Prince Sergei Georgievich Romanowsky, 8th Duke of Leuchtenberg (4 July 1890 – 7 January 1974) was the son of Prince George Maximilianovich Romanowsky, 6th Duke of Leuchtenberg and his second wife Princess Anastasia of Montenegro.
He succeeded his half-brother Alexander Georgievich as Duke of Leuchtenberg from 1942 until his death.
William Montagu Douglas Scott, 6th Duke of Buccleuch
William Henry Walter Montagu Douglas Scott, 6th Duke of Buccleuch and 8th Duke of Queensberry( 9 September 1831 – 5 November 1914) was a Scottish Member of Parliament and peer.
He was the paternal grandfather of Princess Alice, Duchess of Gloucester, and the maternal great- grandfather of Prince William of Gloucester and Prince Richard, Duke of Gloucester.
He was a paternal great- great- grandfather to Sarah, Duchess of York.
Countess Elisabeth of Leuchtenberg
Elisabeth of Leuchtenberg( born: March 1537- died: 6 July 1579 in Dillenburg) was the daughter of Landgrave George III, Landgrave of Leuchtenberg and Margravine Barbara of Brandenburg- Ansbach.
Alexander Georgievich, 7th Duke of Leuchtenberg
Alexander Georgievich, 7th Duke of Leuchtenberg, also known as Prince Alexander Georgievich Romanovsky or less commonly Alexander de Beauharnais (13 November 1881 – 26 September 1942) was the only son of George Maximilianovich, 6th Duke of Leuchtenberg by his first wife Duchess Therese of Oldenburg.
He was a descendant of Paul I of Russia from both of his parents.
Grand Duchess Maria Nikolaevna of Russia (1819–1876)
Grand Duchess Maria Nikolaievna of Russia
(18 August 1819 – 21 February 1876) was a daughter of Emperor Nicholas I of Russia, and sister of Alexander II.
In 1839 she married Maximilian, Duke of Leuchtenberg.
She was an art collector and President of the Imperial Academy of Arts in Saint Petersburg.
Eugen Maximilianovich, 5th Duke of Leuchtenberg
Eugen Maximilianovich Romanowsky, 5th Duke of Leuchtenberg, Prince Romanowsky (8 February 1847 – 31 August 1901) was a son of Maximilian de Beauharnais, 3rd Duke of Leuchtenberg and Grand Duchess Maria Nikolaevna of Russia Duke of Leuchtenberg.
He succeeded his brother Nicholas Maximilianovich as Duke of Leuchtenberg from 1891 until his death.

Question: Who is George Maximilianovich, 6Th Duke Of Leuchtenberg's maternal grandfather?
Answer: Nicholas I of Russia