In Nicaragua, there are only two very small schools, both of them in Managua.
Falcon Flight School, owned and operated by the ex-Director of Civil Aviation has a Cessna 150 and a Piper Aztec on the Aerolub side of the Managua International Airport (MNMG). Telephone numbers are 505-276-0962 (home) and 505-883-0475 (Cel) Ask for Captain José Antonio Bonilla. Last time I heard, rate was around $130/hour dual for the C-150. I have a friend who received the first Private Pilot licence issued in Nicaragua in over a decade who trained there and was satisfied with the instruction received.
Golden Wings Flight School, owned by La Costeña pilot David Enrique Díaz has another Cessna 152 out at the Los Brasiles Airport-- just leaving Managua on the on the highway towards León. Rates are a few dollars cheaper here. Telephone numbers are 505-244-0781 and Celular 505-875-3517.
In Guatemala there are a few more opportunities as well as a considerably more rebust General Aviation scene than in Nicaragua. I flew with an instructor named Ivan Rivas about 10 years ago-- you might want to ask if he is still around. I don't know who rents airplanes currently, but the Aeroclub de Guatemala has friendly folks who can be helpful-- and they have a nice website too! Check it out at
www.aeroclubguatemala.com
Your GFPT sounds a little like the Recreational Pilot Licence issued by the US FAA. Neither are ICAO licences, as categorized by ICAO Annex 1, and therefore would not be recognized in other countries. Private Licences are pretty much similar from one state to another, as ICAO establishes the standards for them for the most part. Once you get one in one country, more often than not, it is an administrative procedure to get your licences validated (recognized) for private flying purposes in other countries.