The ornamental design for gift card box with custom insert, as shown and described.
FIG. 1 is a perspective view of a gift card box with custom insert, illustrating a box cover engaged with the box, and showing my new design;
FIG. 2 is a front elevational view of the invention, illustrating the box cover engaged with the box, the right-side elevational view, the left-side elevational view, and the rear elevational view all being mirror images thereof;
FIG. 3 is a top plan view of the invention, illustrating the box cover engaged with the box;
FIG. 4 is a bottom plan view of the invention, illustrating the box cover engaged with the box; and,
FIG. 5 is a perspective view of the invention, illustrating the box cover disengaged with the box to reveal a custom insert.
The broken line showing a gift card in FIG. 5 is included for the purpose of illustrating a gift card in use with the invention, and forms no part of the claimed design.