How can I delete contacts imported from gmail ?
Open the contacts app and hold the contact name you want to delete then hit delete
To delete them permanently (both at gmail and your android) better do it from gmail.
If you just want to delete from your android and keep them at gmail you should first disable synchronization with gmail (otherwise if you delete at your android they will probably reappear when synching or they will be deleted at gmail too).
Then, you can delete one by one using the contacts (or "people" or "address book") built-in application or better use a specific one. Look for MiniContacts at Google Play store, it allows you to delete any contact easily, even by groups with one click (ie. all those originated by gmail sync).
But remember to disable the synchronization before...
Separate names with a comma.