Well if the light doesn't come back on, then it is not a serious problem. . . If its serious, the CEL will come right back on no matter how many times you reset to turn it off, so If it is a big problem , you will have a chance to check it out as the CEL will be back. . .

As far as the leakage, what was it and where was it coming from? ? ? Replacing a head gasket is a decent amount of work, so be sure that is what the problem is before you do it. . . Just make sure everything is tight. . .

As for the cost, you would have to ask a shop. . . I'm a craigslister myself, found a good mechanic that adjusted my timing and did a valve adjustment for $50, then realized I forgot to put the camshaft seals in and oil got on my timing belt. . . He came back and took everything apart to change my timing belt and pretty much re-did all the work from the previous day as well as put in the seals for another $50. . . I ended up giving him $140 total just because I saw it was a lot of work for what he was charging, but I have not had a problem with his work at all. . .

Just to change the head gasket, you have to remove the head, which is a bitch and once you have it off, you should take it to get a valve job done.

Upgrades for the head are Camshafts, Cam gears, Stems and retainer springs(some say stems are optional as the stock ones work fine), but if you plan to change any of those, buy them and have the shop put them in along with the valve job, because the head has to be off to change the springs.

not sure the difference from GSR to Type-R in the head, but aftermarket like crower camshafts and AEM gears are not hard to find as Type-R stuff is kinda hard to get and will run you about the same, but will be used. . . aftermarket will be new and if you are in California they will come with the C.A.R.B. sticker. . .