Stating a fact, that the wedding was called off due to covid, isn't a sign of bad relationship. Her family stated the wedding had been called off, not that they weren't still planning to be married.
The emotions exhibited by Gabby's parents were after they realized Gabby was missing, the Laundries were refusing to even speak, and later when they found their daughters body. Again, not a sign of any prior issues. In fact, the sheer evidence that the families had been neighbors in NY, and JP had moved down to the area would point to a more probable indication of good relations, including the fact that the families had shared their phone numbers and contact info. And when the kids were first missing, Gabby's family reached out to the Laundries, not simply going directly to LE. Again, an indication that things were civil and cordial at the very least.
While it may be possible they asked Brian where Gabby was and he ran off, that's not probable since by the point he ran off Gabby was already declared missing, LE had been to the Laundrie home and the Laundries had already refused to respond not only to Gabby's parents, but had hired an attorney and refused to cooperate with LE. The actuals in this scenario deny the possibility.