Stripping zinc electroplating with phosphoric acid - hydrogen danger?
Yes, mineral acids like $\ce{H3PO4}$ react with electropositive metals like zinc to produce dihydrogen gas ($\ce{H2}$).

The quantities generated of course depend on how much metal is being dissolved that way. About $65\ \mathrm{g}$ of zinc would produce about $44.8\ \mathrm{L}$ of hydrogen (in Standard conditions).

This may seem like a lot but your nuts and bolts are covered with only a very thin layer of zinc.

Hobby chemists dissolve metals in mineral acids all the time, with minimal danger. Just avoid open flames (use an electrical hot plate) and make sure your work place is well ventilated and you should be fine. Consult any safety advice supplied with the Ortho.