Hi Prashant,
Its very good thing that as you like to learn new things up to date. As per my Experience and as you are a SAP SD Functional Consultant, You can opt for SAP GTS because It is very tightly integrating with SAP SD, MM, FICO, BI, CRM, HANA etc....... So as you said that you have CRM knowledge also right.. So you might be getting a chance to work on GTS by integrating with CRM and SD as well so, your desire will be kept safe ( Now I want to learn a module for which I can work for next 7- 8 years but problem is that I don't want to leave my SD.)
SAP GTS always depends on the Feeder System like ECC modules SD, MM,, FICO so GTS the system obviously needs to get the Data ( Master data, Transaction data) from Feeder system only in view of this, there might be a chance to work SAP SD and GTS simultaneously.
Please share your feelings further if any..
With Regards
Srikanth