Will squeezing lemon juice on top of foods with ferrocyanide additive release hydrogen cyanide gas?
No. The hexacyanidoferrate(II) complex is a very inert complex that does not undergo ligand exchange easily. This is somewhat exemplified by the harsh conditions alchemists used to synthesise hexacyanidoferrate(II), which included high temperatures and strong bases.

Furthermore, whatever amount of lemon juice you add the resulting acidity will be much less than in your stomach. If you swallow a cyanide source such as potassium cyanide, this will get protonated by your stomach acid to liberate hydrogen cyanide and potentially kill you. Food additives — especially approved ones — undergo various testing procedures to make sure that does not happen.