The presentation of the results will depend on the size of values. It is recommended in grams. But it is better to follow the indication of the journal.

if we take the dry weight in grams then is it correct to express the results in mg/g/day

You'd better classify what kind of RGH you need, leaf area ,plant hight or dry matters. Her I share a related document that you may need.

on dry matter basis

The relative growth rate in terms of aboveground biomass (RGR) for each standard plant was calculated as (ln W2−ln W1)/(t2−t1), where W2 is the final biomass, W1 is the initial biomass, and t2−t1 is the time interval (days), respectively. You can refer to the paper of "Effects of phosphorus application on photosynthetic carbon and nitrogen metabolism, water use efficiency and growth of dwarf bamboo (Fargesia rufa) subjected to water deficit"

All depends on the decomposition of TGT in components, that can be related to photosynthesis Anet as Anet/molar surface carbon density with dimension time-1 and molar units,but Lambers et al used mg g-1 day-1.  For reference you may check the presentation of Jarou http://www.slideshare.net/zachjarou/relative-growth-rate-rgr-plant-physiology and the methods in Masle, J., Farquhar, G.D.. and Wong, S.C. (1992). Transpiration ratio and plant mineral content are related among genotypes of a range of species. Australian Journal of Plant Physiology 19, 709-721

thank you very much sir

THANKS FOR ALL