Using String#gsub
with regular expression:
"((String1))".gsub(/^\(+|\)+$/, '')
# => "String1"
"(((((( parentheses )))".gsub(/^\(+|\)+$/, '')
# => " parentheses "
This will remove surrounding parentheses only.
"(((((( This (is) string )))".gsub(/^\(+|\)+$/, '')
# => " This (is) string "