Venus, thanks for the question.  

Ants on gai lan may actually be indicators of another pest.  Cabbage maggots do feast on the roots and ants are there to go after the maggots.  Cabbage flies lay there eggs at the base of brassica plants and once hatched will start feeding on the roots.  Cabbage maggots have been flying and egg laying recently so the timeline seems right.  Unfortunately, once noticed, there is not much you can do.  Crop rotation, cabbage collars, and row covers can help prevent the problem.  Nematodes are another alternative.  Holes in leaves could be flea beetles or cabbage worms. Check out the following link:

• https://extension.umn.edu/yard-and-garden-insects/root-maggots
• https://extension.umn.edu/plant-diseases/insects-and-diseases-cole-crops

I hope this helps.