Is it permissible to say the dhikr: "Glory be to You, and with Your praise, there is no god but You, I seek Your forgiveness and repent to You" upon finishing reading the Qur'an without being in a [specific] gathering?

The wisdom behind concluding recitation with "Glory be to You, O Allah, and with Your praise, there is no god but You, I seek Your forgiveness and repent to You" is to compensate for any shortcomings in it, to perfect the reward, and to thank Allah for His guidance. This applies equally to individual or communal recitation, legislating the conclusion of righteous deeds with remembrance and seeking forgiveness.