To answer your question we must first understand the meaning of the Gemara you quote. Although this Gemara can be understood on many levels I will try to present a simple explanation of the Gemara. Unfortunately, I have had to omit most of the sources and proofs for what I have written.

Adam ha'Rishon was created of two parts, his Guf and his Neshamah. The Guf, being of a physical nature, created from earth, inclines to the aspects of this world and tends to pull a person towards all types of desires and evil. The Neshamah, being of a spiritual nature, tends to spiritual things and tends to pull a person to spiritual perfection. Before the Chet, Adam was on a very high spiritual level, where he was more or less dominated by his Neshamah, as it says "Elokim Asah Es ha'Adam Yashar". When Adam chose to eat from the forbidden fruit, he followed the instincts of his Guf and thus his Guf became more of a dominant feature in his existence. From now on he would live in a constant struggle between good and evil.

When the Gemara says that the Nachash placed in Chavah a Zuhama, it is not necessary to explain this literally. Rather the Gemara wants to tell us that his Guf, which is a physical entity and hence subject to rotting, smelling, decomposition and decay, was now the dominant partner in mankind.

When Klal Yisrael accepted the Torah, they accepted upon themselves to be subject to Hash-m's will and decree, and this willingness enabled them to once again return to the original status of Adam ha'Rishon. The Gemara indicates this by saying Paska Zuhamasam. (The Zohar ha'Kadosh adds that this situation only lasted until the Egel when they once again regressed. However, from our Gemara it is evident that they remained on a higher level than before Matan Torah.)

The other nations, who refused to subjugate their bodies to their Neshamah and rejected the Torah, thus remained on their previous level of subjugation to their bodies' physical desires.

We see therefore, that Geirim, who were not at Har Sinai, remained at the same level as the nations. However, surely the fact that they now come to accept the Torah should help them in the same way that Kabalas ha'Torah aided Klal Yisrael originally.

We can answer this question by examining the words of the Gemara carefully. The Gemara does not say that Kabalas ha'Torah alone was what did the trick for Klal Yisrael, rather that being at Har Sinai was the significant factor. This is because at Har Sinai Hash-m "Revealed his Eish ha'Gedolah" (Devarim 4:36) "In order that Klal Yisrael would have his fear on their faces so as not to sin" (Shemos 20:17). We see that the acceptance of the Torah enabled Hash-m to provide Klal Yisrael with the appropriate lessons to cleanse them from their subjugation to their Guf. Mere acceptance alone would not suffice.

Geirim, therefore, who were not there, should be expected to remain in their previous state. The Gemara replies that even though they were not there, their Mazal was there. In other words, they felt the fear of Hash-m deep down without ever realizing what it was they were feeling (cf. Megilah 3a).

I would argue in light of all the above, that the feeling alone would not be sufficient without the actual Kabalah of Mitzvos and Ol Malchus Shamayim. This is because the fear of Hash-m and realization of His power and the truth of His existence all help a person to subjugate his Guf's will and desires for those of his Neshamah only because he accepts upon himself to strive for this. If the person himself has not decided to subjugate his own desires, all the fear of Hash-m will not help him, since he is still choosing of his own accord to follow his own desires.

Dov Freedman