As long as the pump is still running, it won't trip a gfci. The tank it was in was keeping it cool enough to not short to ground, hence no trip on the gfci. gf= ground fault. It would need to sense a higher amperage than what the mj pump was pulling. The pump wasnt shorted enough yet. Once it got hot enough to trip something, it would have happened either at the gfci or at the breaker for the outlet. In my unprofessional opinion, the chance of fire would have been slim. You would have had to have both a bad gfci and a bad breaker. With that said, I will be replacing my MJ pump tomorrow. lol.
Oh and as for the low volatge dc thing, Ive seen way more power supplies cause fires than pumps that run ac voltage. Power supplies are all ticking time bombs, again in my unprofessional opinion. They rank right up there with ballasts.