migration.js

const GSession = require('./schema')

module.exports = {
    up(dbService) {
        return dbService.createTable(GSession.options.tableName, GSession.schema))
    },

    down(dbService) {
        return dbService.dropTable(Gsession.options.tableName)
    }
}

model.js

const GSession = require('./schema')

module.exports = sequelize => sequelize.define('GSession', GSession.schema, GSession.options)

Schema.js

const { DataTypes } = require('sequelize')

module.exports = {
    schema: {
        id: {
            type: DataTypes.INTEGER,
            primaryKey: true,
            allowNull: false,
            autoIncrement: true,
        },
        start_time: DataTypes.DATE,
        end_time: DataTypes.DATE,
        is_closed: {
            type: DataTypes.BOOLEAN,
            defaultValue: false,
        },
    },
    options: {
        tableName: 'gsessions',
        timestamps: false,
    },
}

Migration files should be immutable while Model schema will be changed anytime. We need to create to new migration file to change db, and then change model directly. So it's not a good idea to share schema between model and migration file.