I'm not so sure about that outside of the powerplay.
Seasons together with Gretzky in LA:
1988-89:
ES TOI (forwards) - Gretzky 16.7, Nicholls 16, Robitaille 15.2, no one else over 13
ES points - G 100, N 87, R 74, no one else over 54 (Tonelli), no other center over 35 (Kasper)
+/- - G +15, N +30, R +5
Conclusion: judging by the +/-, Robitaille likely split time with multiple linemates - though Gretzky and/or Nicholls are by far the most likely culprits by TOI and points
1989-90:
ES TOI - G 17.2, R 14.9, Sandstrom 13.8, no one else over 13
ES pts - G 96, R 67, Sandstrom 43, Tonelli 44, Kasper 39
+/- - G +8, R +8, but Sandstrom -11
Conclusion: Gretzky and Robitaille are quite obviously together, the third line member would be more of a revolving door though, as Sandstrom doesn't fit
1990-91:
ES TOI - G 18.4, R 15.1, Sandstrom 15.2, no one else over 14 (Granato 14, Elik 12.4, Taylor 12.3)
ES pts - G 103, R 68, S 61, no one else over 50 (Elik 50, Granato 44, Taylor 43)
+/- - G +30, R +28, S +27 (Elik +20, Granato +22, Taylor +27)
Conclusion: seems like two clear lines of Robitaille - Gretzky - Sandstrom and Granato - Elik - Taylor
1991-92:
ES TOI - G 16.8, Kurri 14.2, R 14, Granato 13.9, Sandstrom 13.4, rest under 12.3
ES pts - G 63, R 60, Granato 49, Donnelly 42, Kudelski 36, Kurri 30, Sandstrom 29
+/- - G -12, R -4, Kurri -24, Granato +4, Sandstrom -2, Kudelski -15
Conclusion: Gretzky seems to have spent most of his time with Robitaille and Granato and likely some with Kurri and maybe Kudelski. No center beyond Gretzky within anywhere Robitaille's TOI and ESP, though.
1992-93:
ES TOI - R 15.4, G 15.1, Sandstrom 14.2, rest under 13.5
ES pts - R 76, G 38 (in 45 GP), Sandstrom 36, Granato/Kurri/Donnely all around 50, Corey Millen 22 in 42 GP
+/- - G +6, R +18, Sandstrom +12, Granato -1, Kurri +17, Donnely +17, Corey Millen +16
Conclusion: inconclusive mess, thanks to the Gretzky injury. But obviously Robitaille did well without him.
1993-94:
ES TOI - G 16.6, Kurri 14.5, R 13.7, Granato 13, rest under 12.4
ES pts - G 62, R 46, Kurri 31, Donnely 30, rest under
+/- - G -25, R -20, Kurri -24, the only ones near them were 4th liners Warren Rychel (-19) and Dixon Ward (-22)
Conclusion: painfully obvious line of Robitaille - Gretzky - Kurri