Wanted to Limit Restriction in the Approvals

[Santhosh Raj]

We are currently facing some problems in Approvals 

Our Request type has around 4 approvals and we opened the approvals stages from customers, So in this can we able to restrict the gmail users alone from the restrictions

For Example 
In the four Fields, it can show our company domain email address, but restrict the gmail alone from the box, So users can select that can find there approvers.

Customer can create accounts in Gmail, but in approval field they should be rejected.
Is there any way we can able to update it ?