Sept 15 (Reuters) - GMT Capital, the second-largest shareholder in oil company Pipestone Energy, said on Friday it intends to vote against an all-stock deal proposed by larger rival Strathcona Resources as it undervalues the company.

Candian oil producer Strathcona said last month it will go public by acquiring Pipestone to create a combined business with a market capitalization of C$8.6 billion ($6.36 billion).

As per the all-stock deal, which is expected to close in October, Pipestone shareholders will receive 9.05% of the pro forma equity in the combined company, with existing Strathcona shareholders owning the rest.

Investment firm GMT Capital, which holds a 13.47% stake in Pipestone, according to LSEG data, said the buyout deal undervalues Pipestone Energy's common shares.

The acquisition requires approval from two-thirds of Pipestone shareholder votes cast at a meeting in September.

Strathcona and Pipestone did not immediately respond to Reuters requests for comment.

As of last close, Pipestone's shares have fallen 8% since the deal was first announced on Aug. 1. Stock is up 3.6% at C$2.61 in afternoon trading.

Strathcona earlier said the combined company will retain its name and be led by its CEO Rob Morgan.

($1 = 1.3526 Canadian dollars) (Reporting by Tanay Dhumal in Bengaluru)