Hello Sascha,
I have gone through with your requirements that you want a Custom template for e-commerce webshop for Gambio GX3.
Yes, I can build a Fresh & Professional look and feel based Custom template for Gambio GX3 as you want.
I have sound knowledge to work on HTML5, CSS3, Sass, Bootstrap4, JavaScript, and JQuery etc.
I have done many projects that are similar to yours.
++ Let's connect here to discuss more on the detailed requirements of your project.
Looking forward to working with you.
Thanks & Regards,
Aadil Saifi