Does Channeler negate the effects of Arceus & Dialga & Palkia GX's "Altered Creation GX" attack?
It depends on who is playing the Channeler. If the player who used Altered Creation GX uses Channeler then the effects are removed; the player's Pokemon no longer do +30 damage and the player does not get to take any extra prizes from the effect. However, if the player's opponent plays Channeler, then Altered Creation GX effects are not removed as they reside on the other player (who did not play Channeler).
Source: TPCi Rules Team (2020-01-09)

Relevant cards

Note: All applicable cards may not be listed below.
SM11_190
SM11_190
SM11_232
SM11_232
SM12_156
SM12_156
SM12_220
SM12_220
SM12_221
SM12_221
SM12_258
SM12_258