You can easily create a gallery page in the theme by following the steps given below:
Step 1: Firstly, create a new page.
Go to Dashboard > Pages > Add New
You can name the page as a Gallery.
Step 2: Set the template as Gallery Page.
Step 3: Click on Add Media to create a gallery.
Go to Add Media
Step 4: Click on Create Gallery & upload the images from your desktop.
Once the images get uploaded, click on Create a new gallery option as shown in the below image.
After this Edit Gallery page opens up.
Make sure that Link To field should have an Attachment Page option.
Now click on the Insert Gallery.
When you complete the above step, you can see a gallery shortcode appears on the page.
Click on Publish button.