CMPol: I came across same issue and probably found a solution. Bottom line - You need to crack your own game. First download and install GOG version of Summer Challenge. Then download DOS version of the game - for example from Abandonia. Unpack DOS version to any folder You like. After You have done that just copy three files from DOS version folder to GOG version folder:
1/runme. bat
2/sumcrack.exe
3/summer.exe
Now go to the GOG version folder and pick any non-executive file like for example SGDEF.BIN and open it with DOSBOX. That will turn on DOSBOX console. Type RUNME to start a game. Enter setup menu and return to main game menu without any changes. Now You are ready to toss that javelin properly. Spacebar also affect High Jump, Pole Vault, Cycyling and Kayaking so now You can also have fun with those events.
It was testet on Windows 11 but I'm pretty sure it will work on any system. Now You can enjoy Summer Challenge without frustration. My record in javelin is 98.76 - try to beat it!
Thanks, I will have to see if I can get around to trying this out.
I'm very disappointed in GOG though, we should not be required to find workarounds to fully play a game without game breaking issues.