Why is gastric lavage considered to be helpful only if a patient comes to a hospital within 1 hour of a toxic ingestion?
Gastric lavage only washes out ("lavages") the stomach ("gastrium"). Gastric emptying time is generally ~60-120 min, probably on the lower end of that for ingested liquids. After that amount of time, most of the toxic substance will be beyond the level of the pyloric sphincter (i.e. in the duodenum) and not accessible to lavage. 

The second question is not possible to answer without some information about what specific toxic ingestion you are interested in knowing about.