It is often accused that, open source tools are not good for designing and they are absolutely immature comparing to other proprietary tools. But It is not true exactly. There are several open source tools which do well in designing task.
An article published in designer-daily lists 12 great open source tools intended for designing purpose. Some of them are of professional quality are often used in professional design tasks.
In some industries, open-source software is already dominant and leaves little space to proprietary software. In the world of design, it’s harder to make a professional carreer using open-source apps only, I’d go as far as saying that it’s nearly impossible.This said, there are many apps that already allow you to create professional designs, and I can only encourage using those.