How do I convert a latitude/longitude coordinate to a NOAA GFS grid cell location?
** All of the links given with times will need to be adjusted to current dates if reading this at later times or else will appear dead **

Once upon a time I dealt with such files, but not in quite a few years... and Google wasn't the best help at first in finding documentation on what the .ascii is exactly... but thanks to Kevin Goebbert's code here, I was able to find the description page for the model in question is located by removing the .ascii and inputs (and thus here):

And so right now it lists:

Longitude:  
0.00000000000°E to 359.50000000000°E
     (720 points, avg. res. 0.5°)
Latitude:   
-90.00000000000°N to 90.00000000000°N
     (361 points, avg. res. 0.5°)
Altitude:   
1000.00000000000 to 1.00000000000
     (47 points, avg. res. 21.717)
Time:   
12Z07JUL2017 to 12Z17JUL2017
     (81 points, avg. res. 0.125 days) 


The p50 indicates a grid space of only half a degree in that file, so you couldn't actually use your point in this moel.

However, use the 0.25 resolution instead (description):

So first is time first is time, the second is longitude points starting at the Prime Meridian and going east, and the third is latitude starting at the South Pole and going north.  (GFS time steps have always been a mire... but based on this full list of data files and the info page previously given, it indeed appears the data provided here is just every 3 hours for the first 10 days.)

So to pick your value for the second input, it'd be something like  


  ifpositive: (longitude)*4 - 1
  ifnegative: (180+longitude)*4 + 719


And for latitude:  


  360+(latitude)*4


And so if I didn't make a mistake, that'd be this point (lat gridpoint 505, lon gridpoint 1124).