Gedarel is a combined contraceptive pill that contains the active ingredients ethinylestradiol and desogestrel.
It is available in two separate dosages:

20 micrograms of ethinylestradiol and 150 micrograms of desogestrel
30 micrograms of ethinylestradiol and 150 micrograms of desogestrel

The only difference is the amount of ethinylestradiol each pill contains. Ethinylestradiol is a synthetic form of estrogen.
The lower dosage of 20mcg version produces fewer side effects (though this is not the case for all users). In particular, if you suffer from nausea or vomiting when taking a standard 30mcg pill, you should consider the 20mcg. However, higher failure rates have also been recorded with this pill.
Both pills are highly effective contraceptives, working up to 99% of the time.
The lower dosage is also normally recommended for older women.
Due to the hormonal nature of the pill, different ingredients affect everyone differently. Some women may find they respond better to Gedarel 30 and others to Gedarel 20. If you are unsure which is best for you, you should consult your doctor.