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I think it's the LP.

Based on the parent's images and the phenotype of the foal, this is what I can figure the Color Test might show.

Foal: Is clearly Bay Appaloosa. I don't see visible markers for White 2, which his sire carries, or Roan, which his dam carries, although he might carry either without showing them because of he got Llp and PATN1 from somebody. His gene test will probably show Ee aA KitW2 Kit LlpL ECA3P(PATN1)ECA3P+ WF: Minimal. I would guess the white in his tail is from the Appaloosa.

The sire is definitely a White 2 Black. I would postulate a large white factor, because only a little of the black shows on his face. There are a few faint spots on his shoulder and upper loin, so I suspect he is the source of the foal's Llp and PATN1. He may be homozygous for ECA3P(PATN1) since the spots are few, but it can't be proven by the image. The sire's gene test will probably show E? aa KitW2Kit Llp? ECA3P(PATN1)ECA3P? White Factor Large or Extensive.

The dam appears to be a Red Roan with Minimal or Small White Factor. Her gene test will show ee A? KitrKit WF: Minimal (my best guess).

These guesses are all based on the phenotypes (the visual appearance) of the three horses in question. Only running a Color Test on all three will give the definitive answers.

Gulastra Plume won't show up spontaneously, and it has to be homozygous to show. The only way to get it into a line is to GMT it onto a foundation, so unless you have done that or are breeding to someone else's GP horse (and both parents carry it), it is not a possible cause if you get a foal with white in its tail.