In most cases, there is no strict obligation to give a gift when a couple invites you to dinner at a restaurant. The invitation itself is a gesture of hospitality and friendship, and the expectation is generally for the guests to enjoy the company and the meal without any additional requirements.
However, giving a gift as a token of appreciation or to show gratitude for the invitation is a thoughtful and kind gesture. It's a way to express your thanks to the hosts for their hospitality and for taking the time and effort to invite you to dinner.
The decision to give a gift and the type of gift you choose is entirely up to you. It could be something small and simple, such as a bouquet of flowers, a box of chocolates, or a heartfelt thank-you card. Alternatively, you could offer to treat the hosts to dessert or drinks after the dinner as your way of reciprocating their invitation.
It's worth considering the context and the relationship you have with the couple. If it's a close friend or family member, you might be more inclined to give a gift compared to a more casual acquaintance or colleague.
In any case, whether you choose to give a gift or not, expressing your gratitude and saying thank you during or after the dinner is always appreciated and considered good manners. The hosts will likely be delighted to have you as their guest, and your presence and enjoyable company will be valued above all else.