I have had positive experience with 'THE Gayatri' mantra. The one associated with the solar deity 'savitir'. I never did chant any other 'gayatri' dedicated to other deities. My impression was that these gayatri-ized mantras were adapted, perhaps somewhat forced to fit some other deities. So that is my take.
THE gayatri is Sanskrit in origin, but if one is familiar with Sanskrit, I suppose one could compose a new mantra, but I don't know if it will be useful, or even harmful. The sound qualities are important, and I honestly do not know how certain mantras came to me. Some are in other languages, such as Hindi, pali, and more recently in Japanese.
In theory one could even compose a mantra in English. But the sound quality and content is very important, and has to match the overall intended effect of the mantra. One Hindu webpage actually rated English as generating negative energizes. Sanskrit was rated the highest for spiritual vibrations, naturally. I have taken efforts to learn Sanskrit, and while I can't say I am fluent, I will say its a very beautiful flowing language.