Brahms A German Requiem i "Selig Sind"

 In Brahms’s A German Requiem, i “Selig Sind,” the excerpt begins in Db Major, and cadences in measure 96 (an IAC in DbM). In measure 97, with the addition of the Cb, I hear this chord as a 7th chord, particularly a V7 chord in the key of Gb Major, which is closely related to Db Major. I think the piece stays in GbM until m. 102, when it unexpectedly goes to F Major. I think the piece stays in F Major up until the end of the excerpt (m. 110/111), which includes the cadential 6/4 going to a I chord in m 110/111.