The Feyenoord defender appeared to be the first-choice option after starting all three Group B games as the Dutch claimed top spot ahead of Chile, Australia and Spain.
However, the 24-year-old was not involved in the victory over Mexico in the second round as Van Gaal preferred Paul Verhaegh at full-back.
But Janmaat, who confessed to his surprise at being dropped, hopes to convince Van Gaal to restore him to the starting line-up in Salvador.
"It (being dropped) was a blow that I did not see coming," Janmaat is quoted as saying in AD. "But you cannot dwell on it too long.
"I will continue to train continue to train hard, and then we will see. If the coach needs me I will be there.
"The team comes first; we are here with a purpose - that is to be world champions."
Janmaat made his international debut in September 2012 against Turkey, and has gone on to collect 19 caps.
