What is the average daily gain of a pig?

This is going to depend on many factors:
1. Stage of growth
2. Genetics
3. Nutritional program
But if you want pig performance standards that are reasonable for domestic U.S. pigs today, that would be:

Weight range: 10-50 lb.
50th percentile: 0.85 lb.
90th percentile: 1.03 lb/day
Weight range: 50-250 lb.
50th percentile: 1.60 lb.
90th percentile: 1.80 lb/day

From 1999 PigCHAMP records, the following performance was reported:
55-275 lb 1.86 (top 25% pigs)
55-275 lb 1.60 average