Current state of Gene Therapy
The scope of this question is too wide to be answered on Biology SE. However I will give you very brief answers to your questions (as I have rephrased them) and point you towards some sources of basic information on the Internet. After reading these you may wish to return with more specific questions. I have also briefly summarized some of the problems which it is useful to be aware of before reading details of specific approaches to gene therapy,.
Answers

(i) Some approaches involve attempts to replace a defective gene with
a functional gene or add the latter to the patient’s DNA. Others
involve drugs, often nucleic acid in nature.
(ii) Current efforts often have the more modest aim of ameliorating the disease, although there are also efforts to reverse the condition.
(iii) Gene therapy is not yet in general clinical use, although for some diseases (e.g. Severe Combined Immune Deficiency and Chronic Granulomatus Disorder) there have been reports of success in limited clinical trials.

General Reading
I am not an expert in this area, but by Googling for ‘Gene Therapy’ I came up with the following pages that you may find helpful.

Your Genome
University of Utah: Approaches to Gene Therapy
American Society of Cell and Gene
Therapy

You may also be interested to read about the currently controversial drug, Eteplirsen.
Problems in Gene Therapy
Rather than giving you details of technical approaches to gene therapy, for which you may not have the necessary scientific background, I draw your attention to some of the key problems and questions that have to be addressed in this area.

Does one need to counteract the harmful effect of a mutant gene product (e.g. sickle cell haemoglobin), or is it sufficient to produce a ‘good’ gene product to remedy a deficit?
Is it possible to target new cells or does one have to deal with existing mature cells?
(Some cells like blood and immune cells are constantly renewing themselves, whereas others like muscle turn over much more slowly. The latter are more difficult to deal with.)
How can the DNA (or drug) be deliverdd to the cell, and only to the appropriate cell. (A variety of delivery techniques have been employed in different circumstances.)
How can the therapeutic DNA be incorporated into the appropriate position in the genome, and how can one ensure that it is expressed, and that no other genes are affected. (Early attempts at gene therapy using in which DNA was inserted at the wrong position resulted in cancers. However technology here is moving rapidly.)