What is the difference between major medical insurance and guaranteed issue plan?

The two are not necessarily mutually exclusive. Group health insurance from your employer is almost always both.

That said, what you may be referring to is in the individual market you often need to be medically underwritten for major medical which is usually more benefit rich than a guaranteed issue plan which is generally a limited benefit plan.

A limited benefit plan general pays out a specific amount for each covered event regardless of cost. A major medical usually has a catastrophic benefit that caps your expense after which it pays in full. within both there can be a wide range of benefit differences.