Where can I find magnetic declination lookup table?
No, the North pointed by GPS coordinates are extracted using the velocity heading which would represent the true North. If you get the ECEF (earth centered earth fixed) coordinates from the GPS you can convert it to a local ENU (east north up frame) and calculate the heading from the velocity vectors (take a look here if you need the formulas for that). 

However, this site http://www.magnetic-declination.com/ can provide you a estimate for magnetic declination at your location.

EDIT 
From the comment I came across this topic which lead to this

you can play with the code provided and take it as a template for your needs to create a lookup table. The software used for creation of the files is the one present in the site you pointed. Just downloaded the linux zip file and tested it with the sample txt coordinates provided and all seems ok.

For the record the command used was (yes for some reason the extension exe is present but this is linux binary):

./wmm_file.exe f sample_coords.txt out.txt


which created a out.txt equal to sample output file provided. Now this file can be used by the function void build_lookup_table() (present on grab60_char.c) to build the lookup table you need.