The Galaxy (15-5-9) clinched a playoff spot and moved to a first-place tie with the Western Conference rival Seattle Sounders for the MLS lead.
Keane made a nifty stutter move just outside the 18-yard box to get space for the assist.
Gordon had just entered the game in the 75th minute as a substitute for Gyasi Zardes.
Keane tied it in the 62nd minute. Zardes stripped FC Dallas defender Victor Ulloa in the middle of the box and fed Keane for his 15th goal of the season.
Blas Perez opened the scoring for FC Dallas (13-10-6) in the 54th minute with a header off Andres Escobar's cross.
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