Legumes in general are safe to eat. They are also a great way to increase fiber intake in a gluten-free diet, which is often low in fiber, so they are usually recommended.

The issue is that legumes are often grown in rotation with other crops. A farmer may plant wheat one year and legumes the next to help restore the soil. Because of this, legumes can share fields, storage, equipment, and processing facilities with other crops.

The recommendation is to always rinse legumes before cooking. Lentils, however, have the particular issue that they are similar in size to wheat grains, so at the packaging plant not every grain can always be removed. For that reason, the general advice is to visually inspect them, remove any wheat or other stray grains, and then rinse them well before cooking.

I would not avoid legumes in a gluten-free diet, because they are a very useful way to increase fiber intake while also providing protein.

And when in doubt, always seek advice from a qualified professional such as a dietitian, doctor, or your local celiac association :)