Tosafos quotes the Rashbam who explains that נעל וגדר כל שהוא בנכסי הגר does not refer to merely locking the door, because that would not qualify as a chazka to acquire the house, rather it would be מבריח ארי by preventing a damage which doesn't qualify as a kinyan. Tosafos disagrees and proves from other places that locking the door would constitute a kinyan. Tosafos explains that locking the door is more than just preventing damage, because it demonstrates ownership by not allowing anyone else to enter.
Both Tosafos and the Rashbam agree that preventing damage would not qualify as a kinyan, but they argue whether locking the door is just preventing damage or demonstrating ownership. Why doesn't preventing damage qualify as a kinyan? The SM"A (choshen mishpat 192:10) writes:
שסילוק ההיזק אינו קונה, זה לשון הגמ' משום דהו"ל מבריח ארי מכנסי חבירו, פירוש רשב"ם ז"ל הא למאי זה דומה למשיב אבידה דכל ישראל מצווין להציל ממון חבריהם מן ההיזק
The Rashbam holds that since 'mavriach ari' is an obligation on every jew to save the property of another, it doesn't demonstrate ownership and cannot qualify as a kinyan. R' Akiva Eiger points out that based on this rationale, in the estate of a ger who died, since his estate is hefker, the logic of protecting another jews property no longer applies, therefore it should qualify as a kinyan. The Rashbam himself as quoted by Tosafos clearly holds that even in the estate of a ger, protecting the estate does not constitute an acquisition. The Pischei Teshuva quotes from the Ramban that even in the estate of a ger where there isn't any mitzvah, one cannot acquire by merely protecting the property from damage since it is just an act of preventing loss and not an act of a kinyan.