Donations, memberships, and gift cards can be purchased from their respective tabs on your organization page (see image below). The link to your organization page is found in the Settings > Organization Settings page, in the Links area.
Your organization page will have the list of your events and tabs for additional items. Note that these tabs on your customer-facing organization page will only appear if you've created and activated these items, respectively. See the articles below for guidance on creating each of these items.
You may also prompt ticket buyers to add a donation to their purchase at checkout by toggling on the donations prompt in the additional settings of your event. Gift cards can be purchased in any amount when you process an order through the admin.