According to Bates (1), his name is Thomas Gunder, but the company D "Register of men discharged" clearly has 'Thomas Grinder'.
He was 38 years old when he was enlisted (3).
He was enlisted and mustered into service on 26 October 1861 (1, 3, 4). He was enlisted at Philadelphia, Pennsylvania (3). He was mustered in as a private, at Philadelphia, Pennsylvania (3). He served as a private, initially in company H (1).
He was transferred on 4 December 1861 to company D (1, 3, 4).
He was discharged at Washington DC on surgeon's certificate of disability on 8 December 1863 (1, 2 [medical board], 3).
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)
[I did not find an entry for him in the index on <www.ancestry.com> to the 1890 veteran's census (searched May 2007)]