Ah: good news, bad news. First the good: you no longer need two sets of codes, one for international and one for U.S. domestic. Now the bad: those codes are so complicated you can't possibly remember them all. Okay, I can't possible remember them. But even if you can, there is an easier way.
What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.
All clock times are in four figures, UTC.
All estimated elapsed times are in four figures (hours and minutes).
Area preceding Item 3 is to be completed by ATS and COM services.
If called upon to fill out an International Flight Plan you may wish to copy the filed flight plan portion of your computer flight plan as a guide.
7. Aircraft Identification. Normally the aircraft call sign, if used, or the registration marking of the aircraft. (i.e., “TAG5”)
8. Flight Rules. “I” for IFR, “V” for VFR.
Type of Flight. “N” for non-scheduled air transportation operation, “G” for general aviation.
9. Number of Aircraft. Leave blank for one aircraft only.
Type of Aircraft. Designator as specified in ICAO Doc 8643. (i.e., “GLF5”)
Wake Turbulence Category. “H” if aircraft MTOW is greater than 136,000 kg, “M” if less than 136,000 kg but more than 7,000 kg, “L” if less than 7,000 kg.
Left of Slash
Right of Slash:
The minimum to get across the North Atlantic and into Europe is: SXWGHRY/S.
For A Gulfstream G450 with enhanced navigation (Certification F-Enhanced): SBDE2E3FGHIJ3J5M1RWXYZ/HB1D1
The four letter location identifier of the departure airport, or “ZZZZ” if no identifier (then specify airport in Item 18 with “DEP/” followed by name).
15. Speed.Cruising Speed. Enter the “N” followed by four figures to specify true airspeed in knots (e.g., “N0485”), or “M” followed by three figures to specify true Mach Number (e.g., “M083”)
Cruising Level. Enter “F” followed by three figures to specify a flight level (e.g., “F450”), or “A” followed by three figures to specify altitude in hundreds of feet (e.g., “A100”)
Route.Flights along designated ATS routes. Insert the route designator if the departure airport is located on the route, otherwise insert “DCT” followed by the point of joining the first ATS route, followed by the designator of the route. Then, insert each point at which either a change of speed or level, a change of ATS route, and/or a change of flight rules is planned. Followed in each case by the designator of the next route segment, even if same as the previous one, or by “DCT” if the next point will be outside a designated route, unless both points are specified by geographic coordinates.
Flights outside designated ATS routes. Insert points normally not more than 30 minutes flying time or 200 nm apart, including each point at which a change of speed or level, a change of track, or a change of flight rules is planned. Insert “DCT” between successive points unless both points are defined by geographic coordinates or by bearing and distance.
ATS routes. The coded designator (2 to 7 characters), i.e., “UB10”.
Significant points. The coded designator (2 to 5 characters) of the assigned point, i.e., “HADDY”. Degrees only (7 characters) will be two figures describing latitude in degrees followed by “N” or “S” and three figures describing longitude in degrees followed by “E” or “W”, i.e., “46N078W”. Degrees and minutes (11 characters) will be four figures describing latitude in degrees and minutes followed by “N” or “S” and five figures describing longitude in degrees and minutes followed by “E” or “W”, i.e., “4620N07805W”. Bearing and distance from a navigation aid will be described by two or three characters, then the bearing in three figures (degrees magnetic), then the distance in three figures (nautical miles), i.e., “DUB180040”.
Change of speed or level (maximum 21 characters). The point at which a change of speed (5% TAS or 0.01 Mach or more) or a change of level is planned will be followed by an oblique stroke and both the cruising speed and cruising level without a space. I.e., “DUB180040/M082F330”.
16. Destination Aerodrome. The four letter ICAO location identifier or “ZZZZ” if no identifier (then specify airport in Item 18 with “DEST/” followed by name).
Total Elapsed Time. In four digits (hours and minutes).
Alternate Aerodrome. The four letter ICAO location identifier or “ZZZZ” if no identifier (then specify airport in Item 18 with “ALTN/” followed by name).
18. Other Information. As necessary.
For A Gulfstream G450 with advanced NAV: RNVD1E2A1
For A Gulfstream G450 with advanced PBN: B1C1D1L1O1S2
This used to specify the following, but the requirement has since been deprecated.
For A Gulfstream G450 with advanced NAV the answer had been: DAT/SV, but that has since been deprecated. (You don't need it anymore.)
The Gulfstream manuals say to add "RMK/AGCS EQUIPPED" here but you should not do that in a G450/550 because it will prevent your "Acknowledge" reply. For more about this see: CPDLC / Oceanc CLX.
Supplementary Information.Endurance. Four figures (hours and minutes) of fuel on board.
Persons on Board.
Emergency and survival equipment. Cross out those items not carried.
The Gulfstream solution is available here: Gulfstream Flight Plan Codes, but this solution is a bit dated.
Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 21.
Gulfstream ICAO Flight Plan Codes, undated
ICAO Doc 4444 - Amendment No. 1, Procedures for Air Navigation Services, International Civil Aviation Organization, Amendment No. 1, 2007