Scales to Play Over Gdim
Every chord is a subset of several scales: any scale that contains all of a chord's tones fits over it. Pick a chord to see the matching scales from its own root, the major keys the chord belongs to, and full fretboard diagrams for every scale on guitar.
A Gdim chord contains the notes G – A# – C#. Any scale that includes every one of those 3 tones fits over it — from the root G, that means G Blues, G Locrian, G Half Whole Diminished, G Whole Half Diminished, G Gypsy Major and G Gypsy Minor. Start with the first scale listed: it is the most common choice, and the others color the same chord tones differently.
Gdim is also diatonic to one major key: G# major (relative minor Fm). In a song from one of those keys you can also solo with the key's own major scale or its relative minor pentatonic across the whole progression, not just this chord.