GTP protocol is being used to carry IP packets between UE and External network. Since there are different kinds of service possible between UE and External Network, different QoS is applied for different type of data traffics such as FTP, Online Gaming, VoLTE etc. When GTP-C establishes tunnels between eNodeB, SGW and PGW a unique TEID is assigned to each data bearer to carry data of a specific QoS. This how GTP-U is used to deliver different kinds of IP packets between eNodeB and PGW via SGW.
In particular downlink data transmission case, once the eNodeB receives GTP-U packet, it knows which radio bearer will be used for transmission of this IP packet and eNodeB does different treatment for different kinds of IP packets.
I don't know more on GTP removal but I heard on this and going through online available materials. What I learned till now it that ther e is no change in GTP-C (GTP control plane) signalling what is present in LTE network but GTP-U related information will be removed (proposal) from existing GTP-C LTE signalling messages. GTP-U will not be present to send data between eNodeB and PGW rather Ethernet frame 802.1ad will be used for data transmission between eNodeB and PGW using the S-TAGS and V-TAGS. I'm still is in initial stage of learning and will share the more details once I get the complete knowledge of this area.
Hope whatever I covered as above in brief, it will make sense to proceed further.