Hi there 
From your description, it sounds like you want customers to earn 1 free gift for every 5 products purchased, with the number of available gifts increasing automatically as they add more items. You also want the gift selection experience to be smooth, so customers can immediately choose all eligible gifts without having previously selected gifts being removed and re-selected.
A setup like this can be handled with Easify Box Bundle Builder BYOB.
I’ve created a similar demo to help you better visualize how it works:
First, you can go to Discount & Subscription and create your Box Size Options. For example, you could create box tiers such as 5 products, 10 products, 15 products, and 20 products. Each box size represents the quantity threshold that unlocks additional gifts.
Then, in the Free Gifts section, you can configure your Free Gift Rules. For a 5-item box, you can unlock 1 gift. For a 10-item box, unlock 2 gifts. For a 15-item box, unlock 3 gifts. For a 20-item box, unlock 4 gifts. The app can automatically increase the number of available gift selections as customers move to larger box sizes.
Using a setup similar to the demo below, customers can clearly see how many gifts they’ve unlocked and select the correct number of gifts based on the quantity they’ve purchased. This creates a much smoother experience than some gift apps that remove previously selected gifts whenever a new threshold is reached.
So if your goal is essentially “Buy 5 get 1 gift, Buy 10 get 2 gifts, Buy 15 get 3 gifts, Buy 20 get 4 gifts”, while allowing customers to select their gifts without constantly redoing previous selections, Easify Box Bundle Builder should be a solid option to explore 