You can sort photos in a gallery in a few different ways:
By Filename, which is alphabetically based on the name of the file.
By Capture Time, which is when the file itself was created or the image was captured (note, if this option isn't showing up, the capture time could have been stripped out of the file on export).
By Upload Date, which is the date and time the image was uploaded at.
For each of the options, you can select to sort ascending or descending.
To sort photos in a Gallery, click the three dots under the "Upload Photos" button while editing the Gallery, and choose "Sort Photos".
Why are my images out of order when I sort them alphabetically?
Sprout acts like most photo browsers and file managers when sorting alphabetically - it sorts by the first number it finds in the filename. Therefore, if you name your files like this: Image-1, Image-2, Image-3 ... Image-10, Image-11, etc. Then Sprout will sort them with Image-1 first, and then Image-10 because it sees the 1 in 10 first, and 1 comes before the 2 in Image-2.
This is why we recommend naming your files with a zero-padding so that your filenames will look more like this: Image-001, Image-002, Image-003 ... Image-010, Image-011, etc. When you name your files this way, Sprout (and every other photo browser and file manager) will sort them as you'd expect because it sees the zeros and can put them in the proper order. Make sure you have the proper amount of zeros in comparison to the number of images you're exporting. For example, if you have 1000 images, you'll need your images to have 3 zeros before the filename (Image-0001).
How to rename files properly in Lightroom
To rename files with zero padding (as described above) in Lightroom:
Select all of your photos.
Click Library > Rename Photos.
From the "File Naming" dropdown, click the "Edit" option.
Remove any text in the editor.
Insert "Custom Text" from the bottom "Custom" section.
Type in a dash (to separate your custom text from the sequence).
Insert "Sequence # (0001)" from the "Sequence and Date" section.
Save this as a preset. Now, when you rename files, you'll be able to type in a custom text (i.e. the client's name), and then it'll automatically zero-pad your numbers.