Prasnop
The answer is yes. Technically, you can use the same ATP group for products which will undergo ATP check in ECC / APO. You just need to activate ATP check integration model for a product-location combination to differentiate whether the check will be in ECC or in GATP. So, yes - you can use the same ATP group for a product which needs a ECC Av. Check and another one which needs a GATP check.
However, I would like to understand why you would like to create such a complex scenario. I would suggest you to use different ATP groups to be assigned to products which will undergo ECC and GATP checks. This way you will directly be able to identify which products are GATP relevant just be looking at the ATP group and there won't be any confusion.
I hope that answers your query.
Parth Soneji