1. 无理由的；不必要的；无缘无故的
If you describe something as gratuitous, you mean that it is unnecessary, and often harmful or upsetting.

1. without cause

e.g. a gratuitous insult

2. unnecessary and unwarranted

e.g. a strikers' tent camp...was burned with needless loss of life

Synonym: needlessuncalled-for

3. costing nothing

e.g. complimentary tickets
free admission

Synonym: complimentarycostlessfreegratis(p)