gem 'grape-axlsx'

use Rack::Config do |env|
  env['api.tilt.root'] = '/path/to/view/root/directory'
end

class API < Grape::API
  content_type :xlsx, "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
  format :xlsx
  formatter :xlsx, Grape::Formatter::Axlsx
end

resources :users do
  get axlsx: 'users/user.xlsx.axlsx' do
    @users = User.all
  end
end

# or you can omit .axlsx extension

...
get axlsx: 'users/user.xlsx' do
...

# NOTE: xlsx_package is predefined instance of Axlsx::Package
wb = xlsx_package.workbook
wb.add_worksheet(name: "Users list") do |sheet|
  sheet.add_row ["id", "First name", "Last name", "Middle name"]
  @users.each do |user|
    sheet.add_row [user.id, user.first_name, user.last_name, user.middle_name]
  end
end