To add the gallery on the front page please follow below given steps.
1. From Customize section, go to Widgets > Front Page Widget Area.
2. Add widget “Gallery” to Front Page Widget Area. Note: Gallery widget is only available from WordPress version 4.9.
3. Now, click Add Images and start adding Images to the gallery according to your requirement.
4. After choosing the required image for gallery section set the size and column as per your requirement. Also, do not forget to select option Media File in Link To Options.
5. If the preview looks fine, save the changes by clicking on publish button.
You have to upload the image of uniform size for the best result.
Note: Do not forget to regenerate your thumbnails using the plugins given below after choosing the desired images :