After activating the theme, there'll be suggested plugin listed at the top (if there aren't, you can go to 'Appearance > Install Plugins' section as well ).
Try installing and activating these following plugins ( as it effects the importing process ). You can also install all suggested plugins at this step as well.
Then, go to 'Appearance > Gecko Sample Data > Choose Layout > Import Demo Data'
Follow the steps mentioned on the screen, then, you'll get the site like the demo 🙂
Can't import demo sample date
You can add below code in function.php line 2 in themes/gecko/function.php and try import again. This problem of hosting limit memory. After import complete you can remove code.