Of course Jarrod "trolls" to always lash out after he's assumed a "fact set".
I was referring to
1) that the OP did NOT indicate if this was a low-voltage pump, 110/120 or 220/240 pump. "pond pumps" are to this day commonly LOW voltage 24 or 36 Volts are sold.
2) The OP indicated that he also recently installed a new timer, and that the GFCI "goes off" when the pump is not in use, hence the suspected incompatibility or error in wiring regards to the TIMER and GFCI.
3) the OP did NOT indicate if this was a submersible OR an EXTERNAL pond pump.
4) The OP did NOT indicate an installation of a componant GFCI, a receptacle GFCI or a GFCI circuit breaker, I assumed NOTHING.
5) GFCIs are rated for use in specific power circuits at specific ratings. Choosing the wrong GFCI for your application, or improper installation can affect your situation.
6) The OP did not indicate if the pump was a wet drive, magnetic drive or direct drive pond pump. The different drive systems can have their own host of trouble shooting factors if the tripping is not a wiring or defective GFCI issue (compatibility issues).
7) There are a multitude of factors and options that could effect this situation and a host of component and wiring errors that could have taken place; there are TOO MANY UNKNOWNS as the OP did NOT specify adequately to make ANY judgement WHATSOEVER. Jarrod has assumed a "fact set" that has NOT been presented, including what he PRESUMED what I was referring to in my previous post. As usual, his suspicions were WRONG.
8) The OP indicated ONLY that he installed a NEW GFCI and NEW timer, but did NOT indicate if he was REPLACING OLD GFCI and OLD timer, or introduced for the first time either or both of these features to his "pond pump" circuit. Again, I assumed NOTHING, but other posters, Jarrod included, have.
I strongly suspect Jarrod uses other posting names, posts ambiguous subjects, then answers himself and belittles others because of a fact set he ASSUMES, and is NOT presented by the OP (original post).