To log GTP profile drops, you need to configure the logging settings for the GTP profile on your Juniper device.

Below are the commands to configure logging,

set security gprs gtp profile gtp1 log forwarded detail---GTP packet was forwarded because it was valid.

set security gprs gtp profile gtp1 log state-invalid detail-- GTP packet was dropped because it failed stateful inspection or a sanity check.

set security gprs gtp profile gtp1 log prohibited detail--GTP packet was dropped because it failed message length, message type, or IMSI prefix checks.

set security gprs gtp profile gtp1 log gtp-u all--All GTP-U packets were dropped

set security gprs gtp profile gtp1 log gtp-u dropped--GTP-U packets that were dropped.

Log messages as below will be seen, 

RT_GTP_UPDATE_USER_TUNNEL_V1: update gtpv1 user tunnel, nsapi 5 from sgsn 10.0.0.1/000131ef ggsn 1.1.1.1/9462bc13 to sgsn 20.0.0.1/0000b311 ggsn 113.187.3.73/9462bc13

RT_GTP_V1_TNL_DELETE_PDP: gtpv1 message delete user tunnel successfully, teardown:0x40, nsapi(5), control tunnel(10.0.0.1/0x1d033c58 -> 1.1.1.1/0x7250b8e2)

RT_GTP_V1_TNL_CREATE_PDP: create gtpv1 primary user tunnel(10.0.0.1/0x00019331 -> 1.1.1.1/0x94711385, nsapi(5/5)) successfully, control tunnel(10.0.0.1/0x1d033c58 -> 1.1.1.1/0x7250b8e2)