I have a simple question.
When we create a custom service in BLE GATT configurator, it asignes a random UUID. Is this UUID a valid unique ID or must we go https://www.guidgenerator.com/ to obtain a valid one ?
You can use the automatically generated UUIDs as such. The GATT configurator generates random UUID values (Version 4 UUID, see: https://en.wikipedia.org/wiki/Universally_unique_identifier )
The UUIDs that you use must not overlap with the Bluetooth base UUID, defined in the Bluetooth Core specification: xxxxxxxx-0000-1000-8000-00805F9B34FB.
When using random UUIDs, you don't need to worry about overlapping with the Bluetooth reserved UUID values. The random values generated by GATT Configurator (or www.guidgenerator.com if you want to use that) are not guaranteed to be unique, but in practice the probability of having a 128-bit random UUID that matches the UUID in some other product is astronomically small.