Not from a gasket, it leaks where the main shaft fits through the case. If you look at the trans below the front bearing retainer there is a hole that the shaft sits in,on the case,thats where it leaks
The main shaft is the one the clutch goes on the one below it is the cluster. if its the cluster there rteally is no way you can fix it without pulling it apart. like someone else said just wipe it with thinner and smear some ultra black silicone on it. other than that the trans would have to be completely dissasembled.
With the shaft out you can use a center punch and tap around the outside of the hole on the case and basically glue it back in. What your doing is actually making the hole smaller and then the pin will fit tight when put back in.
If its leaking the case is getting worn out (they oval there badly). If you have a good guy local to you and can take it apart get the case bored and sleeved so you have the correct press fit with the countershaft then its over and fixed right..
If your looking for a cheap fix I have seen many attempts at this with silicone , snot, JB weld , beat and peening and really its all a bandaid and will just come back to get you eventually.
Oversize parts (shafts) I have not seen because the shaft needs to go through the Countershaft from the back and there's a step in the back of the shaft so you can't make the front bigger cause you can't get it through the needle bearings.
The cheeziest I ever got was to bore an internal O ring groove into a slightly worn bore of the case then when I slid the C/S through it supported and sealed it.
Unfortunately......if it is the countershaft bore in the case, you need a new case. Auto Gear makes a replacement case with a welch plug provision and they also offer the required shorter countershaft. If the input shaft is the source of the leak, well there is no seal. The bearing retainer nut and the release bearing collar are a mechanical seal,(metal on metal), they both require replacement at the same time due to galling. Also, while the box is apart, replace the aluminum mid plate with a cast iron plate. This will stop the hammering effect and subsequent wear that the countershaft/case has to endure.