Gambhir led the way with 81, Karthik made 76 and Dravid was 57 not out as India ended the first day of four against the English county champions well-placed on 281 for three.
Dravid, dropped early in his innings, had added an unbroken 75 for the fourth wicket with VVS Laxman (37 not out), at the close.
Sussex, as has been common practice for English counties for more than 20 years in matches against touring teams, were without several rested senior players in captain Chris Adams, Zimbabwe batsman Murray Goodwin and Pakistan bowlers Mushtaq Ahmed and Rana Naved.
Karthik and Gambhir put on 129 for the second wicket following the early loss of Wasim Jaffer.
Karthik then had the distinction of becoming Saqlain Mushtaq's maiden first-class wicket for Sussex.
The former Pakistan off-spinner, whose British passport arrived earlier in the week, bowled Karthik to end a 177-ball innings featuring 10 fours.
Gambhir's patient display ended when he was caught and bowled by Sussex stand-in skipper Michael Yardy, who arrived late after being released from the England squad for the third one-day international against West Indies earlier in the day.
Dravid was reprieved on 11 when he struck Yardy hard to short mid-wicket where Chris Liddle dropped the catch.
India's captain went on to complete an 82-ball fifty with three fours.
The opening match of India's three-Test series against England is due to start at Lord's on July 19.
Brief Score (at stumps, D-1):
India 281 for 3 (Karthik 76, Gambhir 81, Dravid 57 n o, Laxman 37 n o; Martin-Jenkins 1 for 29, Saqlain Mushtaq 1 for 47, M Yardy 1 for 33) Vs Sussex.