The "gapOffset" parameter is being present inside MeasGapConfig IE, which goes as part of MeasConfig to an UE.
gapOffset may have either gp0 or gp1.
1. gp0 ranges from 0 to 39.
2. gp1 ranges from 0 to 79.
By using the value of gapOffset, UE finds out the SFN and subframe of that SFN to perform measurement.
I have written the action taken by an UE once it receives RRC ConnectionReconfiguration for measConfig.
1> if measGapConfig is set to setup:
2> if a measurement gap configuration is already setup, release the measurement gap configuration;
2> setup the measurement gap configuration indicated by the measGapConfig in accordance with the received gapOffset, i.e., each gap starts at an SFN and subframe meeting the following condition:
SFN mod T = FLOOR(gapOffset/10);
subframe = gapOffset mod 10;
with T = MGRP/10 as defined in TS 36.133 ;
2> release the measurement gap configuration;
Hope I have answered your query :) .