Do I really need to replace head gasket?

What is your question?

I have a 1972 BMW model 2002, not the tii,your system didn't have my year and make. Compression 140 on all 4, there is white residue on spark plugs. Mechanic says I need to replace head gasket.What's your take on these symptoms? What is the average cost of head gasket replacement? What do you think of the Product "Steel Seal" instead of replacing the gasket? Thank you very much for your knowledge and help. Respectfully, Phoenix