Suitable GPU for Convolutional neural network

I have to train a CNN using alexnet with a data set of approximately 300 images. I currently use mac book OS EL capitan with 1,3 GHz Intel Core i5. I wnat to plan in advance so would I need a GPU? If yes which one will be suitable for these configurations?

[Walter Roberson]

El Capitan is no longer supported by current MATLAB versions. Also the latest available gpu driver for El Capitan is 8 series but R2018b requires 9.1 and r2018a required 9.0

[FSB]

I am running matlab R2018a on EI capitan. If I update it to Mac OS mojave, will it solve the issue?

[Walter Roberson]

R2018a was not supported on Mojave: that needs R2018b or later.

[FSB]

Thank you. Another possible configuration that I can use is windows 7 intel i3. What kind of GPU would be recommended?

[Walter Roberson]

Up through R2018b support Windows 7 SP1. I see that CUDA 10.0 supports Windows 7, so earlier versions would as well. CUDA Capacity 3.0 is required as of R2018a.

You appear to have a 2017 MacBook Retina, such as a MNYN2LL/A (Rose Gold). Those have Intel HD Graphics 615 but no NVIDIA GPU in them. There has been discussion about adding external GPUs to MacBook Pro but it is doubtful that you would be able to get reasonable performance between your MacBook and an external GPU through your single USB-C connector: the MacBook is not designed for those performance levels.

A Windows 7 system with i3 processor is probably not designed for anything more powerful than an NVIDIA M series or GT series, something like a GTX540 at the outside.

If you need to do non-trivial CNN work, I would suggest you consider a much newer system.