The following technique can be used for filling gaps in a single time series returned from the query q:

sum(q) or vector(0)

The q is wrapped into sum() in order to drop all the labels for a time series returned from q, so the time series can be matched with vector(0) time series according to matching rules for or operator. The query without sum(): q or on() vector(0) would return two time series on the graph instead of a single time series: one time series from q with its own set of labels and another time series with zero labels and with zero value where the q time series contains gaps.

Unfortunately Prometheus doesn't provide an easy ability to fill gaps with zeroes if q returns multiple time series with distinct labelsets. Other Prometheus-like solutions such as VictoriaMetrics (I work on it) provides default operator for this case. For example, the following MetricsQL query fills gaps with zeroes for all the time series returned from q:

q default 0