According to Bates (1), his name is Thomas Gunder, but the company D "Register of men discharged" clearly has 'Thomas Grinder'.
He was born in 1822/23 (3 (38 in 1861)).
He enlisted and was mustered into service at Philadelphia, Pennsylvania, on 26 October 1861 (1, 3, 4). He was a private in company H (1, 7).
He was transferred on 4 December 1861 to company D (1, 3, 4).
On 28 January 1864, he applied successfully for a pension (5, 6).
1 Bates, Samuel Penniman. History of Pennsylvania volunteers, 1861-5. Harrisburg: B. Singerly, state printer, 1869-71. 5 volumes. 'Ninety-first regiment', volume 3, pages 186-233. (In the roster) (Thomas Gunder)
2 company D, register of men discharged, entry 8 (Thomas Grinder [sic])
3 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 9 August 2004 (Thomas Gunder; roll has 'Thomas Grider')
4 company H, register of men transferred (Thomas Grinder [?])
5 pension index, by name (Thomas Grinder)
6 pension index, by regiment, 91st PA Infantry, company D (Thomas Grinder)
7 index to compiled service records of volunteer Union soldiers who served in organizations from the state of Pennsylvania (Thomas Grinder)