How to decrease front teeth gap without braces?

Depends on gap size: Depending on how large the 'gap' is between your front teeth, you may be able to have veneers placed or full coverage crowns. However, an esthetic workup is needed to see how the restorations would look prior to treatment. You would not want to do this if the gap space is significant, as you'd not have good symmetry and esthetics. It the space is too wide, orthodontics of some type is needed.

Clear brases: If the reason you don't want braces are the brackets, then you might want to consider clear braces like invisalign.

MANY options: You can get veneers, lingual braces, invisalign, crowns may even be an option if your teeth have been heavily damaged in the past. Ask your dentist what is best for you. Dont worry about being an adult with braces, very common now!

Lots to consider: Is the space hereditary, or did it form over time due to shifting of your teeth? Do you have your posterior teeth, or has your bite collapsed? How large is the space? Condition of the front six maxillary teeth? Any periodontal issues? These questions need to be addressed by your local dentist, who may suggest invisalign.

Don't limit yourself: If you really care about your dental health, what will look the best & what will stand the test of time, please don't limit yourself as to your options. It is impossible to advise you what is best over the internet. You need to see a well respected dentist to step back with you, consider the issues as described by dr. Davantzis, & who without a professional bias, can discuss what is best for you.

If between 2front : You may have a muscle attachment called a frenum which would need to be reduced prior to any type of orthodontic tooth movement. Think of this muscle like a rubber band, if you try to push the teeth together, this "rubber band" will prevent that from occurring. See a dentist for an evaluation and a discussion of your options.