“went down westward.” This is correct. The border is now described as going down in elevation from the top of the watershed to the Mediterranean Sea.
“Japhletites.” This is another smaller tribe or clan on the southern border of Ephraim, which is the northern border of Benjamin.
“and on to Gezer.” Again, this description, “from Gezer to the Sea” is fairly vague. Perhaps it more or less followed the Aijalon Valley/Nahal.
That the tribe of Ephraim was given the city of Gezer is geopolitically significant since Gezer controlled the main highway route to Jerusalem and thus access to that important city which would later become the capital of the monarchy under David and Solomon.
“the sea.” That is, the Mediterranean Sea.