What kind of overflow is that? Just a generic box? If so, as mentioned above, those typically need two gaskets, one on each side of the glass. That is the way the Glass-Holes overflows and many of the ghost style overflows are installed. Like this...
Showing gaskets on both sides of the tank wall (wood template shown in photo).
If joining slip PVC connections, use only PVC cement. I usually use cleaner, but many people have success without it. There is a new product by Oatey called Fusion, which has cleaner and cement combined in one step. I am using that on my new build.
Many people tell you to not use teflon tape on PVC. They recommend using a non-hardening joint compound. While that may be the preferential and recommended way to go, if you have ever used it, you will know that it is also the messiest option. It never dries and will end up everywhere. Bill Wann, who builds commercial systems and very large private aquariums, uses silicone on the screw together joints. Just remember that the fittings are tapered and are not meant to screw in all the way. If you use teflon tape or silicone, be sure to screw in the fitting hand tight and then no more than two complete turns with channel locks or a strap wrench. I have been in the hobby since the early 80s and have always used the teflon tape without issue.