Sounds like it's gone, the mechanism is when the turbo inccreased the pressure it forces gassses through the gasket, you can carry out a sniff test to see, but i suspect it will come back positive. you'll then have two options, sell it as is, which will be prob 4-5k, or have the engine rebuilt which will be anywhere from 4k upwards.
The fix is an engine rebuild, theres no shortcuts and anyone advising anything else is giving you bad advice, dont put 'head gasket sealant' or anything like that in as it will prob junk the whole engine including the block. Assuming it is headgasket theres a hgih probability the bearings will have been pinched and deformed, so its litteraly a full strip down and rebuild of the whole engine. The full extent of the damage wont be known till is stripped, again be very wary of anyone telling you any different, untill its stripped an assessed then theres no way to know the engines condition, i know of engines that have been found to have ovaled the bores so bad the blocks have had to be binned.
Assuming you do go down the rebuiild route then its a good opertunity to solve the inherant problems of the 2.5, decent pistons, decent head gasket, decent bearing etc. You'll also need to decide how far you go with the rebuild. If your planning on keeping the car and tuning it in the future then its worth looking at higher spec pistons, higher spec headstuds, maybe even closed decking the block, but that really comes down to how far you want to push it.
I went the whole hog with mine, closed deck, forged pistons, up rated rods, uprated head (valves, vale springs etc),
have a nosey to get some ideas of whats involved.
Also i would advise finding a specialist your happy with, tell them what you want, 400bhp, built for a future 500bhp etc etc and let them spec it, build it, fit it and map it. Means your not gonna get caught out if something fails and various companyes blame each other.
Personaly i use scoobyclinic up at chesterfield for all my work and cant recomend them highly enough, but there are plenty of specialists out there.