Then you have your answer. Her corrected age is actually three months less than the actual age as she needs to catch up to her actual age first. Technically she is now a six month old and for six months her weight is fine. If she is growing steadily, is happy and active then just relax. Please do not listen to what people have to say.

she was 6 month prematured her weight was 0.870grm and current weight is 7 kg

Please share more details like her birth weight height and current weight and height. It is not possible or practical to try and measure weight gain every day.