8va/8vb is an instruction to the player about where to play the notes in relation to what’s on the page, not an indication of where the notes sound in relation to what’s on the page.
(my apologies if what I’m about to write is remedial for you):
Many instruments are what are known as ‘transposing instruments.’ Most of these, for example, a Bb clarinet, sound at a different pitch than they are written. So, a C on the page for a clarinet is actually sounding a Bb. You don’t indicate on the clarinet’s music that the notes written are different than the actual pitch that comes out of the instrument.
What key an instrument is ‘in’ (ie, the pitches that sound when the instrument plays a written C scale) has to do, usually, with the pitches/overtones that sound when the instrument is fully open (brass), or the most simple scale that can be played by opening holes in the instrument in order.
Some instruments are transposing, but only for ease of legibility ; to reduce the need for ledger lines. Guitar is a transposing instrument that falls into this category (as is double bass). But, like the clarinetist who is reading in his ‘home key,’ the guitar player’s music is in her ‘home key.’ There is no need to indicate what the absolute pitches are.
However, if you’re preparing a score, the story may be different. There are two types of scores, a ‘transposed score,’ in which each instrument appears in its own key, and a ‘non-transposed score,’ in which all notes for all instruments appear in the same key. In this case, you would/could use 8vb on the guitar staff, if you wanted to avoid ledger line use.