By far, the majority of GPU farms mine Ethereum. This mean any GPU-mineable coin other than Ethereum is trivially vulnerable to majority attacks. The only thing preventing these attacks from happening is the financial incentives against performing such a purely destructive attack.
ASIC-mineable coins are safe from such attacks from GPU miners.
People point out that the high barrier of entry into ASIC manufacturing can lead to monopolies, such as Bitmain which is a quasi- (not quite) monopoly with its 70-80% market share in Bitcoin with the S9. But first and foremost, manufacturing monopolies don't matter to the function of Bitcoin. A manufacturing monopoly isn't a hashrate monopoly: Bitmain cannot attack Bitcoin. They themselves only directly own <10% of the hashrate.¹
¹ Not to be confused with Bitmain's mining pools which represent more than a third of the hashrate. People usually misunderstand the function and nature of mining pools. End-users choose to mine at whatever pool they want. If Bitmain's pools started sending malicious mining jobs to stratum clients, the community would react and the end-users would promptly abandon them. So in practice Bitmain cannot do whatever they want for however long they want with their pools' hashrate. They enjoy the privilege of representing this hashrate, and this privilege could evaporate overnight.