GS use patched PEAR (extlib).
This is related issue (this is main repository).
https://notabug.org/diogo/gnu-social/pulls/293
I think it may solve this issue by appending following code on head of lib/util/framework.php.
// Avoid conflicting with standard PEAR. set_include_path(__DIR__ . PATH_SEPARATOR . get_include_path());I will try it on this weekend. Please wait. If you want to fix soon, I recommend use PHPv7.4.
If you want to join discussion, please join chat or mailing list (https://www.gnusocial.rocks/, this is current official site.). I check every day.