Based on recent information furnished by RSA Member and author Don Ware (the absolute Guru on this model), it is estimated that your revolver was produced in December of 1864. This estimate is based on monthly deliveries to the Ordnance Department plus and allowance for defective revolvers. Don Ware published the results of his decades long research in the book "Remington's Army and Navy Revolvers 1861-1888".
The cartouche "GP" is attributed to Giles Porter, a Principal Sub-Inspector for the Ordnance Department. Porter seems to have served at Remington on more than one tour of duty during the War, as the serial number range of the revolvers bearing his cartouche are spread out from 20,000 to over 100,000.
The other markings you describe are from unidentified in process inspectors.
New Model Army Serial Number 116991
Ordnance Dept. Monthly
Rcvg. Date S/N Range Qty Model Contract Date
DEC.1864 113460 124350 9000 NMA Nov. 21, 1863
While information concerning the issuance of specific Remington firearms is not available, RSA members have been treated to wonderfully detailed articles in the RSA Journal over the years and some back issues are still available. You may wish to check the web site to see which issues contained articles related to the New Model Army revolver and if copies are still available.
Based on the photos you supplied, this is a very nice specimine of the NMA revolver and would be a proud addition to any collection.
Hope this helps,
Mike Strietbeck