Steps -
1. go to 'xampp/php/' folder in your computer, open 'php.ini' file
2. press CTRL + F to search for 'gmp', you will find this (follow screenshot)
Just remove the semicolon (red marked in screenshot) & save the file.3. if the xampp is already running, then restart the xampp.
4. then navigate to 'core' folder of your project in command prompt & run 'composer update' command.
Hopefully, It will work now.