The GPs in Singapore are certainly empowered to treat common psychiatric conditions like anxiety and depression. Yes, they can prescribe antidepressants. Psychiatry is an important part of the medical education and all of them have been trained to treat common psychiatric conditions like anxiety and depression.
You can look at the credentials of the GPs in your neighbourhood. Many of them would mention that they have a diploma or certificate in psychiatry or mental health issues. For those who mention this in their clinic sign board, they are comfortable with prescribing antidepressants. You can also ask the counter staff at the clinic if the clinic has any particular antidepressant. Most GPs are familiar with antidepressants like Fluoxetine and Escitalopram.
If you are already seeking treatment from a psychiatrist, you can also ask your psychiatrist to recommend a GP for you. The advantage of seeking help from the GP is that many of them do run clinics in the evening and that is indeed very convenient for patients to seek help.
Cheers!
