There are no gaskets or seals in the area you circled. The adapter into the injector is a tapered thread connection, and the connection to the hard injector line is accomplished by a compression flare.
I would take every connection apart, clean everything carefully, and reassemble with the upper end of a proper torque value.
It may just the compression connection to the hard injector lines, you might try just snugging these up a bit to see if that solves the problem. However, do not use an open-end wrench for this; obtain the proper size flare nut wrench. Do not over-tighten. While it is difficult to describe the proper torque by feel, yo fill find that even with moderate pressure the nut will simply stop moving. Tighten no more!
ALL of this assumes perfectly clean flares. If they have been loose for some time, debris can accumulate between the male and female flares. No amount of tightening will solve this properly. Back them off completely and disconnect, check and clean, and then reassemble.