IIT Guwahati has released GATE 2018 response sheets of candidates.IIT Guwahati will publish officila answer key also in next few days.

Meawhile you can check various coaching institute answer key below:
GATE Answer Key
Also, Use GATE College Predicotr to check your chances in IIT, NIT and PSUs.
GATE College Predictor

The official GATE 2018 Answer Key will be released shortly at www.gate.iitg.ac.in . Visit this site & enter required credentials i.e. Enrollment ID, Registration Number and Date of Birth. GATE CE Answer key will be displayed on your screen. Check and download it for future reference.