To what extent were Gorbachev's reforms responsible for the fall of the Soviet Union?

To blame Gorbachev alone is an oversimplification. Reforms may have accelerated the decline of the USSR, but their failure was contingent on long standing economic conditions.
Firstly - what were Gorbachev's reforms, and why did he implement them?By the mid 1980s, the USSR was mired in economic difficulty, with a 1982 GDP 32% that of the US. The country was ill-equipped to deal with the arms race initiated by Reagan's administration, as its scarce resources were already overstretched by the war in Afghanistan. Moreover, the economic difficulty derived from deeper, structural problems; the inefficiency of a centralised and overly-bureaucratic communist apparatus meant that the USSR was on the brink of collapse.Gorbachev advocated reform in order to minimise the likelihood of a revolution.Glasnost - increased openness in government, low-level elections to take place.Perestroika - the introduction of marginally capitalist elements into the economy, such as decentralising industry to individual managers and working on an individual incentive scheme.International reforms - Gorbachev recognised that an arms race would bankrupt the USSR, and so the country had to pursue rapprochement. A number of conferences ensued suggesting gradual nuclear disarmament.
These reforms all had the same goal - to save the USSR. They failed to do so because the subjects they sought to reform structures were intrinsically untenable. They accelerated decline - by renouncing the Brezhnev doctrine, Gorbachev allowed for the rise of nationalism in Eastern Europe where it previously would have been crushed. The reforms were not, however, responsible for the decline of the USSR. If the reforms hadn't been implemented, the USSR would likely have been bankrupted by the arms race with the US.
Questions like this involve a fair bit of conjecture - what would have happened under different leadership? This can help shed light on the period in question by elucidating specific forces that are obscured by surface understandings.