Kindly see the following works and codes (commands) on gravity model for stata. I am very sure these codes will help you a lot.

(1) Stata do file for Gravity Model

https://www.researchgate.net/.../275018402_Stata_do_file_for_Gravity_Model

(2) Hands-on gravity estimation with STATA - AGRODEP

www.agrodep.org/sites/default/files/Technical_notes/AGRODEP-TN-05_2.pdf

(3) Notes on Gravity Regressions in STATA - Jack Rossbach

http://rossbach.georgetown.domains/teaching/spring2016/eco445/hw/STATA_Instructions.pdf

(4) Calculation of the gravity model in R and Stata software: why are coefficients the same but standard errors different?

https://stats.stackexchange.com/questions/201812/calculation-of-the-gravity-model-in-r-and-stata-software-why-are-coefficients-t

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Goodluck

thanks sir it may help me a lot