Adding JIRA gadgets to Confluence is covered by Atlassian documentation. Here's a list of references to get you started.
Structure Gadget may allow modification of structure, updating and creating issues under the account that is used by Confluence to access JIRA. Make sure you understand how Trusted Applications work before allowing production structures to be accessed with this kind of authentication. Using OAuth is more secure because the end-user will never be able to do anything that they are not able to do directly in JIRA.
Main article: Adding JIRA Gadgets to a Confluence Page