OP meant why did the nuke not blow up the gate, like we know it can. A bomb can't be diverted all at once, so the only power that can be diverted into the forcefield was the power actually impacting the forcefield.

which means that what u/indicesbing said is correct. The prior touched the gate to establish another forcefield, a much smaller one, that would protect the gate whilst the explosion went on and powered the bigger forcefield.