Hi @azbilly ,
To make Gallery show all items by default, set OnStart/OnVisible property: Set(Var_Letter, "");Set(Var_Search, "")
To rest the gallery:
1. Set OnChange property of search text box: Set(Var_Search,'Search Box'.Text)
2. Modify Items property of Gallery:
Filter(Search('Test Glossary List', Var_Search, "Title", "Definition"), StartsWith(Title,Var_Letter))
3. Add a Rest button, set OnSelect property: Set(Var_Letter, "");Set(Var_Search, "");Reset('Search Box')
Test Result:

Sik