How can an external observer explain the gamma ray burst of two merging black holes if he never sees them merging due to time dilation?

Question

 Let us assume two black holes moving in direct collision course.  From far away, due to time dilation, an external observer would  perceive the relative motion of the two black holes slow down and asymptotically freeze before the two event horizons touch. However, we assume that the merge of two black holes will result in a gamma ray burst, as perceived by an external observer. Aren't these two notions contradictory?

Comments

Voting to close as unclear.

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Too many misconceptions.
1a) Horizon: it is ill-defined in wild dynamical situations such as a BH merger, see quasi-local horizons.
1b) Horizon: an observer at infinity does not "see" the horizon, he sees a shadow instead, which is larger than the horizon. Considering also 1a), you simply have to compute what happens on a case-to-case basis.
2) Gamma-ray bursts: these are generally not expected to emerge from "bare" black hole mergers. The source of radiation is usually assumed to be matter outside a black hole (its horizon).