Hi everyone, i have a 2005 GMC sierra it has about 370,000 miles still running strong but it is starting to leak oil from the engine i was thinking on doing overhaul if so how much does all that going to cost me?

You can either replace the engine or rebuild the one you have. If you replace it with a rebuilt engine most shops will exchange the core for a discount. That's usually the cheapest and fastest way to get a new engine. Otherwise pull the engine and break it down and let a machine shop determine how much work is needed. You can try to get by with just a basic rebuild kit (new gaskets and seals), or you can go all in and rebuild it. Rebuilding usually starts with a dip in hot caustic soda to clean and degunk the block, then magnaflux to check it for cracks. Then the degree of machining will have to be determined (checking bores for round, decks for warping, etc.) With 370K I would be surprised if it didn't need some degree of machining, but then again you might just have a case of a blown rear main seal, which might be repairable without even pulling the motor.