An engagement party is not considered as a gift giving event. Generally, only those very close to the couple or the families bring a present. However, it is not mandatory. The couple would also not be expecting any gifts at an engagement party.
You may instead buy gifts for the couple at their bachelor/bachelorette party, bridal shower; and more importantly, the wedding. Sometimes, the couple might follow a "No gifts" policy. A gift should not be taken if the couple has mentioned so. However, consider taking a gift in case you are not going to make it to the wedding or the other occasions.
If you are feeling awkward to go without a gift, you may want to choose a suitable one. The gift should be small and sweet. You may want to save high-ticket presents for the more important occasions.
You should make it a point to meet the couple in person and congratulate them. The couple should put the gifts aside, and give importance to the guest's presence. This will certainly help avoid embarrassment for those guests who did not bring gifts.