Gowdy reportedly met with White House chief of staff Mick Mulvaney at the White House on Tuesday.
Initial reports said that Gowdy had declined to join Trump's legal defense team, but those stories were later recanted.
However, Gowdy is not officially joining the Trump administration — he will serve only as outside counsel.
Gowdy's decision to join Trump's defense team comes after the White House sent an eight-page letter to top House Democratic brass on Tuesday officially responding to the impeachment probe.
In the letter, White House counsel Pat Cipollone called the impeachment probe an "illegitimate" inquiry in which the White House will not participate.