Step 1: Background of Round Table Conferences.
The British government organized three Round Table Conferences in London between 1930 and 1932 to discuss constitutional reforms in India.
Step 2: Participation of Gandhiji.
- First Round Table Conference (1930): Congress boycotted, so Gandhiji did not attend.
- Second Round Table Conference (1931): Gandhiji attended as the sole representative of the Indian National Congress, following the Gandhi-Irwin Pact.
- Third Round Table Conference (1932): Congress again boycotted, and Gandhiji did not participate.
Step 3: Conclusion.
Thus, Gandhiji attended only the Second Round Table Conference as a Congress representative.
\[
\boxed{\text{Second Round Table Conference}}
\]