1.5 cm gravel for a planted tank would be way to big for it to be successful, it might be ok for some plants but it wouldnt be ideal, you should really try and get gravel around 2-3 mm and mix it with laterite.
Gravel by itself is inert and does not contain any benificial minerals that plant roots need, the best substrates for planted tanks are Seachem Flourite, Eco Complete or as i have said small gravel mixed with laterite, but the first two would be preferable.
If you only use gravel by itself you would probably have to provide root tabs and that may work for you, but smaller gravel is still preferable.