GM Forge is described as 'From Steam:' and is a Virtual Tabletop in the category. There are more than 10 alternatives to GM Forge for a variety of platforms, including Windows, Online / Web-based, Mac, Linux and Android. The best alternative is Roll20, which is free. Other great apps like GM Forge are Foundry Virtual Tabletop (Paid), MapTool (Free, Open Source), Fantasy Grounds (Paid) and Astral Tabletop (Freemium).
Roll20 is the free, easy way to play tabletop games (such as pen and paper RPGs) online with others. It's completely web-based, so it runs right in your browser. It supports video and voice chat, background music, character sheets, a massive art library, and more!
Foundry VTT is a standalone application built for experiencing multiplayer tabletop RPGs using a feature-rich and modern self-hosted application where your players connect directly through the browser.
MapTool is an online, multiuser, networked, graphical, interactive, programmable virtual tabletop. Requires Java 6+. It can be used for face-to-face play, or Internet role-playing in real time even if your players are on the other side of the world.
Standard Action is a tabletop gaming application designed to be simple, but flexible. In game, Standard Action features an interactive game board, live chat, dice rolls, a turn tracker, customizable token stat bars, and much more.
Beyond Tabletop is a set of tools for RPG players with easy-to-use online character sheets and maps. Subscription-free and designed to be used on all devices online. Currently offering a battle map and character sheets for Pathfinder and D&D 5e.
Rolisteam helps you to manage tabletop role playing games with remote friends/players. It provides many features to share maps, pictures and it also includes tool to communicate with your friends/players.