Must one wear a belt for prayer?
It is learned from the verse (Amos 4:12) “Hikon Likrat Elokecha Israel” (lit. “prepare yourself to meet your God, O Israel”) that one must prepare oneself for prayer. From here, the Shulhan Aruch (O.H. 91:2) rules that one must wear a belt to separate one’s heart from one’s reproductive organs before praying. In some communities, a special belt, known as a Gartel, is worn prior to praying.
Rabbi Ben Zion Abba Shaul (Or Lezion, vol. II, ch. 7, § 13) mentions several reasons why adding an extra belt is not necessary. Firstly, people in previous generations wore Jellabas or other robe-like clothing which may have appeared unkempt, and thus needed to be gathered together with a belt before prayer. Nowadays, however, people’s attire is generally well-fitted and does not require a belt to look orderly. Secondly, there are different items which people normally wear which can act as a separator between one’s upper and lower body, such as a normal belt, underwear and an undershirt.
The Mishna Berura (M.B., O.H. 91:11) adds that one should not pray in a the clothes with which one sleeps, that is, pyjamas.
Summary: One should pray in a decent manner, which includes the way one dresses. One need not wear a Gartel.