Hangul syllable galb

No; 갋 has no fixed meaning outside a Korean word.

It is a Hangul syllable block made from an initial ㄱ, vowel ㅏ, and compound final ㄼ, itself written with ㄹ+ㅂ. A jamo is one Korean letter; Hangul arranges letters into a syllable block. The label “galb” describes that construction; it is not a translation. Compare 갈, with final ㄹ, and 갉, with final ㄺ.

Hangul was devised so Korean could be written without requiring Chinese-character literacy. Its earliest historical record appears at the end of 1443; an explanatory work in 1446 set out its principles. It became Korea's official national writing system in 1894. A study of Korean textbooks from the Enlightenment period reports that ㄺ and ㄼ were the only double finals consistently kept as clusters, while other clusters were simplified or written separately.

How do I type 갋?

With Korean input enabled, type r k f q on standard 2-beolsik.

Those keystrokes enter ㄱ, ㅏ, ㄹ, and ㅂ in sequence; a Korean IME combines ㄹ+ㅂ into final ㄼ, then composes 갋. These Latin letters name key positions on that layout; they are not a romanized spelling of the character.

Other Korean layouts or romanization-based input methods use different keystrokes.

Why can the final ㄹㅂ sound change?

The block's final letters do not fix its pronunciation.

In Korean, ㄼ is a written compound final whose sound depends on the word and what follows. A 2013 study of Seoul/Gyeonggi university students found that speakers commonly realized ㄼ as the sound associated with ㄹ in the tested contexts; the following syllable and word-specific exceptions still matter.

Read 갋 as part of the full Korean word. The isolated syllable block does not settle how its final is pronounced.