The Endpoint Block Plus Wrapper does not allow access to the GTP_DUALs. You might need to modify the following GTP ports depending on your board:
Starting with v1.6.1 of the Endpoint Block Plus Core, there are options in the CORE Generator customization GUI allowing you to adjust these values. The script can still be used if you want to adjust the values on the routed NCD file to potentially speed up debug efforts.
Download the Perl script "BP_GTP_mod.pl" from:
Also, for more information on these signals, please refer to the "Transmitter Output Swing" and "Transmitter Pre-Emphasis Settings" tables of the Virtex-5 RocketIO GTP Transceiver User Guide (UG196 v1.4, page 114):
08/04/2008 - Corrected release version to v1.6.1 instead of v1.5.1
04/29/2008 - Updated to include that values can be changed in GUI starting with v1.6.1.