What game completion % are you currently at?
If you can complete some of the Special Conditions, European/Japanese/American Hall or Manufacturer Events to boost your completion to 50%, you'll be awarded the
Jaguar XJR-9 Race Car '88 which should make mincemeat of the GTWC Event.
Alternatively you can complete Missions 30 - 34 to win
Nissan R89C Race Car '89 another vehicle which should annihilate the GTWC.
Your AMG should be a match for this event, but it depends whether you'd like a challenge, or just want to get through the event for few A-spec points to unlock the Extreme Hall.
Take a look here for AI lineup information for the GTWC event.
The lineups are consistent regardless of what car you enter, after a console reset. Lineup 6 is an excellent challenge, but the hardest lineup available, whereas lineup 10 is the easiest. Bear in mind that the Toyota GT-One and Nissan R390 GT1 are flipped in the PAL / NTSC releases, so it depends where you are in the world. Also the big Group C cars (Sauber, 88C-V, R92CP, etc) all pit during races, whereas later cars (GT-One, Speed 8, 787B, etc) do not.
As has already been suggested, the El Capitan 200 miles Endurance is the place to win the
Minolta Toyota 88C-V Race Car '89 which'll pulverise GTWC opponents! It's an easy race to win even in an unmodified car, as the list below, showing cars & drivers that've done this for full 200 A-spec points shows:
Endurance races: El Capitan
Car: Lotus Esprit Turbo HC '87 (Tires F/R S2/S1) / MR / 228HP (oil changed, 794.16 miles on car) / 1146kg / 5.0kg/HP /
200 A-spec by Toronado
Car: Honda S2000 '04 (Oil changed, S1 tires) / FR / 259HP / 1250kg / 4.8kg/HP /
200 A-spec by LeadSlead#2
Car: BMW M3 '04 / FR / 339HP / 1570kg / 4.6kg/HP /
200 A-spec by TurboJ
Car: Ford Mustang SVT Cobra R '00 (used,no oil) / FR / 367HP / 1628kg / 4.4kg/HP /
200 A-spec by bhawk776
Car: Suzuki GSX-R/4 / MR / 173HP / 640kg / 3.7kg/HP /
200 A-spec by daan, pupik, foghorn_100 & alhajoth
Car: Toyota Supra RZ '97 (Used, oil changed, SM tyres) / FR / 325HP / 1510kg / 4.6kg/HP /
200 A-spec by Smallhorses
Car: Honda NSX-R Concept '01 (S1 tires) / MR /
200 A-spec by jdw
Car: BMW M3 CSL '03 (oil changed) / FR / 378HP / 1385kg / 3.7kg/HP /
200 A-spec by mprata
Car: Opera Performance S2000 '04 / FR / ?HP (274HP on garage screen) / 930kg / ?kg/HP (3.4kg/HP using garage value) /
200 A-spec by CBR250RR & Tired Tyres
Car: Aston Martin Vanquish '04 / FR / 436HP / 1835kg / 4.2kg/HP /
200 A-spec by FLK
Car: Jaguar XKR R-Performance '02 (S2 tyres, no oil) / FR / 396HP / 1735kg / 4.4kg/HP /
200 A-spec by speedy_samurai & Thirdeye
Car: Opera Performance S2000 '04 (S1 Tires) / FR / 274HP / 930kg / 3.4kg/HP /
200 A-spec by Licence Test
Car: Honda NSX Type-R '02 (oil changed) / MR / 310HP / 1270kg / 4.0kg/HP /
200 A-spec by dkstz
Car: Amuse S2000 R1 / FR / 259HP / 1120kg / 4.3kg/HP /
200 A-spec by marekd
Car: Lotus Motorsport Elise '99 (S1 tires) / MR / 204HP / 700kg / 3.4kg/HP /
200 A-spec by torque998
Car: RUF 3400S '00 / MR / 306HP / 1300kg / 4.3kg/HP /
200 A-spec by fuzzydice
Car: Acura HSC '04 / MR / 330HP / 1150kg / 3.5kg/HP /
200 A-spec by M2R
Car: Amuse S2000 R1 (no oil) / FR / 259HP / 1120kg / 4.3kg/HP /
200 A-spec by Snyp
Car: Mitsubishi Lancer Evolution VI RS T.M.E '00 (Used, oil changed, S1 tyres) / 4WD / 297HP / 1260kg / 4.2kg/HP /
200 A-spec by Vash666
Car: Pontiac Solstice Coupe Concept '02 (S2 tires)/ FR / 243HP / 1318kg / 5.4kg/HP /
200 A-spec by alhajoth
Car: Holden Commodore SS '04 (S2 Tyres) / FR / 328HP / 1658kg / 5.1kg/HP /
200 A-spec by Smallhorses
Car: Mazda RX-7 Type R Bathurst R (FD) '01 (Used, oil changed, S2 tyres) / FR / 290HP / 1260kg / 4.3kg/HP /
200 A-spec by Smallhorses
Car: Honda Dualnote '01 (Acura DNX in NTSC) (S1 tires, oil changed) / 4WD / 410HP / 1400kg / 3.4kg/HP /
200 A-spec by alhajoth
Car: Honda NSX Type-R '92 (oil changed) (S2 tyres) / MR / 283HP / 1230kg / 4.3kg/HP /
200 A-spec by Tired Tyres
Car: Nissan Skyline GT-R M-spec Nur / 4WD (S2 tyres) / 341HP / 1580kg / 4.6kg/HP /
200 A-spec by Tired Tyres
Car: Chevrolet Corvette Grand Sport (C4) '96 (S2 tyres) / FR / 335HP / 1496kg / 4.5kg/HP /
200 A-spec by Tired Tyres
Car: RUF RGT '00 / RR / 379HP / 1330kg / 3.5kg/HP /
200 A-spec by Rotary Junkie
Car: Lotus Esprit Turbo HC '87 (S2 Tyres, no oil) / MR / 215HP / 1146kg / 5.3kg/HP /
200 A-spec by Smallhorses
Car: Mitsubishi Lancer Evolution VI RS '99 (6.2mile used, no oil) / 4WD / 312HP / 1260kg / 4.0kg/HP /
200 A-spec by alhajoth
Car: Lotus Esprit V8 '02 (S1 tires) / MR / 353HP / 1380kg / 3.9kg/HP /
200 A-spec by alhajoth
Car: Nissan 350Z (Z33) '03 (S2 tyres) / FR / 282HP / 1320kg / 4.6kg/HP /
200 A-spec by ferrari_chris
Other alternatives:
Win Fuji 1000km Endurance to get
Nissan R92CP Race Car '92
Win Sarthe I 24hr Endurance to get
Audi R8 Race Car '01
Win Sarthe II 24hr Endurance to get
Bentley Speed 8 Race Car '03 which was my weapon of choice for GTWC, which you can read about here:
Another GTWC report. Stock Bentley Speed 8.
This should provide you with useful info on what to expect regarding laptimes, and pit strategies for each of the GTWC races. 👍
Hope this helps and good luck!