Double up on embedded quotes

With .Columns("G:G")
    .FormatConditions.Add Type:=xlExpression, Formula1:="=AND(MATCH(A:A,Guests!A:A,0),G:G>TIMEVALUE(""6:00""),G:G<>"""")"
    .FormatConditions(1).SetFirstPriority
    With .FormatConditions(1).Interior
        .PatternColorIndex = xlAutomatic
        .Color = 255
        .TintAndShade = 0
    End With
    .Range("G1").FormatConditions.Delete
End With