Anto,
You have a gasket wherever one part of an engine is bolted to another, the gasket forms a good seal and makes sure there are no leaks.
Of all these gaskets the biggest is the head gasket. The engine is in two halves the bottom half or 'block' that the pistons go up and down in hast the top half or 'head' bolted to it.
This is a very important join, coolant , oil gasses all are kept in place and seperate by this gasket. If it fails coolant and oil can mix, and get into the combustion chambers.
Often when the head gasket fails the car will overheat and unless that's spotted quickly and you stop driving it the head can warp and will no longer fit the block properly.
You've seen that the gasket itself is cheap - the problem is that fitting it is a big job. The oil and water need to be drained, the timing belt will be removed (and almost certainly replaced), exhaust, fuel injection and other ancillaries disconnected and the head removed. Chances are the head will be slightly warped and have to be sent to an engineering shop to be skimmed (a thin layer of steel removed to make it flat again) and the whole thing put back together again with a new head gasket.
Shop around for quotes though - my nephew recently neede a head gasket replacement on a 16v Astra and most quotes were £8-1200 but a local garage quoted £4-600 and they did a brilliant job - you may not be that lucky but it pays to shop around.
When you ask - ask them whether the quote includes having the head skimmed if it's warped to avoid someone quoting you a low figure and then saying - oh sorry the head needed to be sent away
Best of luck