I imagine it's too late for the original purpose now, but the correct answer is this.
1) Technically, it's a trick question, as there isn't a diatonic diminished seventh chord in any major key; the dim7 chord's natural habitat is the minor mode.
2) However, it's long been common to use dim7s in major, so we'll presume that the most common case--where the chord is built on scale degree 7--is what's intended here.
3) Scale degree 7 in Gb would be F. So, building a 7th chord on F gives us F-Ab-Cb-Eb. However, that isn't a dim7; it's a half-diminished 7th instead. (Sometimes this is referred to as a "minor-7-flat-5.") So you have to lower the Eb to Ebb ("E double flat.") The chord spelled out: F-Ab-Cb-Ebb.
4) Second inversion means that the fifth of the chord (Cb) is in the bass. The order of the chord tones above that doesn't affect inversion.