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my 68 t120 is leaking oil from the headgasket on the left cylinder. The motor is rebuilt, and runs good. Compression is at about 140psi.
It seems the leak just started. Im aware that some people mention that the inner headbolts can cause oil to leak past the threads and migrate out towards the headgasket.
What approach should I take to fix this issue? Should i remove the head and install a new annealed headgasket or just retorque the headbolts?

It is more likely to be a leaking pushrod tube. There are no oilways through a triumph head gasket

I think it is possible for oil to go past the head socket nut washer, down the stud and out the gasket. Careful cleaning and watching for the leak might give a clue.

im certain its coming from the headgasket. I can visibly see it wet from the headgasket and migrating its way down each fin on the cylinder.

Hello; I had that problem in my 79 but the head surface was not good between the chambers (where the 5/16 studs are) BUT the pushrods tubes had worn out wrong O rings so could be leaked a bit from there too.
I dismantled changed the rocker box gaskets for the cov seal gaskets; cooked the head gasket and re installed; put the thicker washers under the internal 3/8 hex sockets nuts; put new O rings. Checked the head "crush" (keep it less than 0.050. I had about 0.030) and torqued to the Manual figures (did not had that)
The head still was not good but no more heavy leak so possibly was more from the tubes than from the head; however; I have a persistent small drop from the rocker feed (I changed and cooked the copper washers but without any luck) the rear one drop migrates to the fins...is very little and I clean it but if I let it could be confusing at some point.
I did 9000kms with that.

1968 is radically simpler than a 1979. Not the same head design at all.

The base gaskets ONLY on the rocker boxes need re-gasketing. That's it. Simple job. Done in several hours.

DO NOT remove the outer head bolts or the cyl head.

I removed the rocker boxes and the gaskets are def the cause. Problem is I received the wrong gaskets fro my bike.

Would there be any issues leaving the rocker boxes off and four inner headbolts out for a few days till the gaskets came in? Like heads warping or headgasket sealing issues?

No.

On re-assembly I would torque the inner 4 head bolts that pass through the rocker boxes. Then, 1 at a time, I would loosen each of the outer 4 head bolts and re-torque them. Loosen only (maybe) 1/2 turn and re-torque.

Wouldn't hurt to check the rocker box sealing surface is flat. Stick some 120 grit to a flat surface and wipe the sealing face across it just once and you will quickly see any low spots.