From the ResearchGate FAQ:

Changing your login email address

It is possible to use a deactivated email address to log into your ResearchGate account as long as you can remember the password. This is how you can change the email address associated with your ResearchGate account:

1. Log in to ResearchGate using your current login email address 2. Click on your profile image on the top right-hand side of any page 3. Click Settings 4. Under Email address, click Add additional email 5. Type in the email address you want to use as your login 6. Enter your password and click Add 7. A confirmation email will be sent to your new email address. Click the link in this email to complete the verification process. If you encounter an error message "Oops! We couldn't find that token in our database", please make sure you are using the link in the latest email, as all previous links are invalidated. Check your spam folder and filters if you can't find any others. 8. Then go back to your Account Settings 9. Select Set as primary next to the email address you've just added to make it the primary email address for your ResearchGate account. You can now use this email address when next logging into ResearchGate.