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.