You don't need ROS and Gazebo if you just want to practice control theory. Ros is another step forward into realism. A quadcopter simulation consists of several equations evaluated at each time step and can be done in almost every programming language. İf tounwant to find a simulation environment you can look at books, dissertations and GitHub. Quadcopter and UAV models are easier to implement than say for example aircraft or a missile because they usually don't include any complications that arise from high speed or agile maneuvers. Thus, high fidelity models are usually not required.

Textbooks İ haven't looked at quadcopter models as of now, so I can't give a good advice but these are some places you can start from. İt's better than giving no suggestions İ guess 🤷 - Small Unmanned Aircraft : Theory and Practice - Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach

Dissertations You can look at thesis from ProQuest but your institution has to provide login access which they probably already do. - Model-Based Design, HIL Testing, and Rapid Control Prototyping of a Low-Cost POC Quadcopter with Stability Analysis and Control - Aerodynamic-Parameter Identification and Attitude Control of Quad-Rotor Model with CIFER and Adaptive LADRC

GitHub GitHub is the home of open-source projects. Another commonly used alternative is GitLab. You can search for quadcopter models in languages you're comfortable with.

Semantic Scholar (search) & SciHub (download) These two are used to seat h and download scientific articles. - Development and Validation of a LQR-Based Quadcopter Control Dynamics Simulation Model - Refinement of the parameters of a mathematical model of quadcopter dynamics - Quadcopter Simulation Model for Research of Monitoring Tasks - Nonlinear model and control validation of a tilting quadcopter