Gboard: Set Custom Unicode Characters on Longpress or Add Custom Keyboard Keys or Layout

On Gboard, when I long-press the "a" character, for example, I can see some other "a" characters pop up which have different marks or diacritics on them. By using long-press, I am able to easily type the ā and Ā characters, but we don't have such an option for "h" character for instance, as I would like to similarly type the ḥ and Ḥ characters. And similarly, I would like to be able to type some other Unicode characters like ʿ by long-pressing the ' key. These characters are accepted in standard academic Arabic romanizations and copy-pasting these Unicode characters from other applications is very inconvenient. - Is it possible to be able to add my own characters to the long-press of the current characters on Gboard? - Or is it possible to create my own keyboard layout and add that to Gboard (like we add new languages and each language has its own key-set)? - If not, is there a way to hack-edit the language source file (when we download a language in Gboard) and assign my custom keys to them, so that when that language is set in the Gboard, it has my custom keys set in it? - Or if it is not possible on Gboard, then is there any other keyboard app that allows us to customize its keys like this? - If it is not possible on Gboard, how to suggest a custom-long-press-key-setting feature to the Gboard team? Thanks.