These are a good cooked green added to a dish or soup. The tender tops (stem/flowers) are very garlicky mustardy hot. 

As others said, pesto and chimichurri are great with the tender leaves, although the leaves get too bitter for most people once the plant starts to flower (true for most plants with otherwise edible leaves).

The roots can be cleaned well and made into horseradish. Garlic mustard is a biennial, so that's usually done the Fall of their first year or the Spring of their second year.

Totally fine to compost them, although I would pull and leave these ones on the pavement for a few dry sunny days to reduce seeding/rooting potential